<?xml version="1.0" encoding="us-ascii"?><InstanceReport xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"><Version>2.4.0.8</Version><ReportLongName>060500 - Disclosure - CUSTOMER CONCENTRATION AND RISK</ReportLongName><DisplayLabelColumn>true</DisplayLabelColumn><ShowElementNames>false</ShowElementNames><RoundingOption /><HasEmbeddedReports>false</HasEmbeddedReports><Columns><Column FlagID="0"><Id>1</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

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</LabelSeparator><Level>1</Level><ElementName>us-gaap_RisksAndUncertaintiesAbstract</ElementName><ElementPrefix>us-gaap_</ElementPrefix><IsBaseElement>true</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText /><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell></Cells><ElementDataType>xbrli:stringItemType</ElementDataType><SimpleDataType>string</SimpleDataType><IsTotalLabel>false</IsTotalLabel><UnitID>0</UnitID><Label>CUSTOMER CONCENTRATION AND RISK [Abstract]</Label></Row><Row FlagID="0"><Id>2</Id><IsAbstractGroupTitle>false</IsAbstractGroupTitle><LabelSeparator>

</LabelSeparator><Level>2</Level><ElementName>us-gaap_ConcentrationRiskDisclosureTextBlock</ElementName><ElementPrefix>us-gaap_</ElementPrefix><IsBaseElement>true</IsBaseElement><BalanceType>na</BalanceType><PeriodType>duration</PeriodType><IsReportTitle>false</IsReportTitle><IsSegmentTitle>false</IsSegmentTitle><IsCalendarTitle>false</IsCalendarTitle><IsEquityPrevioslyReportedAsRow>false</IsEquityPrevioslyReportedAsRow><IsEquityAdjustmentRow>false</IsEquityAdjustmentRow><IsBeginningBalance>false</IsBeginningBalance><IsEndingBalance>false</IsEndingBalance><IsReverseSign>false</IsReverseSign><PreferredLabelRole>verboseLabel</PreferredLabelRole><FootnoteIndexer /><Cells><Cell FlagID="0" ContextID="c20121001to20130630" UnitID=""><Id>1</Id><IsNumeric>false</IsNumeric><IsRatio>false</IsRatio><DisplayZeroAsNone>false</DisplayZeroAsNone><NumericAmount>0</NumericAmount><RoundedNumericAmount>0</RoundedNumericAmount><NonNumbericText>&lt;div style="font-family: Arial; font-size: 10pt;"&gt;&lt;div style="text-align: left; font-family: Arial; color: #000000; font-size: 10pt; font-weight: bold;"&gt;&lt;!--Anchor--&gt;NOTE 5&amp;#8212;CUSTOMER CONCENTRATION AND RISK&lt;/div&gt;&lt;div&gt;&lt;br /&gt;&lt;/div&gt;&lt;div style="text-align: justify; font-family: Arial; color: #000000; font-size: 10pt;"&gt;We derive a significant portion of our revenue from a limited number of government Clients. Typically, our contracts with these Clients allow them to terminate all or part of the contract for convenience or cause. We have one Client, the Internal Revenue Service, or IRS, which is the source of more than 10% of our revenues from Payment Solutions operations.&lt;/div&gt;&lt;div&gt;&lt;br /&gt;&lt;/div&gt;&lt;div style="text-align: justify; font-family: Arial; color: #000000; font-size: 10pt;"&gt;The following table shows the revenues specific to our contract with the IRS:&lt;/div&gt;&lt;div&gt;&lt;br /&gt;&lt;/div&gt;&lt;table cellpadding="0" cellspacing="0" style="width: 100%; font-family: Arial; font-size: 10pt;"&gt;&lt;tr&gt;&lt;td valign="bottom" style="padding-bottom: 2px; vertical-align: bottom;"&gt;&lt;div&gt;&amp;#160;&lt;/div&gt;&lt;/td&gt;&lt;td valign="bottom" style="padding-bottom: 2px; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;td colspan="6" valign="bottom" style="border-bottom: #000000 2px solid; vertical-align: bottom;"&gt;&lt;div style="text-align: center; font-family: Arial; color: #000000; font-size: 8pt; font-weight: bold;"&gt;Nine months ended June 30,&lt;/div&gt;&lt;/td&gt;&lt;td nowrap="nowrap" valign="bottom" style="text-align: left; padding-bottom: 2px; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;/tr&gt;&lt;tr&gt;&lt;td valign="bottom" style="padding-bottom: 2px; vertical-align: bottom;"&gt;&lt;div style="text-align: left; font-style: italic; font-family: Arial; color: #000000; font-size: 7pt;"&gt;(in thousands, except percentages)&lt;/div&gt;&lt;/td&gt;&lt;td valign="bottom" style="padding-bottom: 2px; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;td colspan="2" valign="bottom" style="border-bottom: #000000 2px solid; vertical-align: bottom;"&gt;&lt;div style="text-align: center; font-family: Arial; color: #000000; font-size: 8pt; font-weight: bold;"&gt;2013&lt;/div&gt;&lt;/td&gt;&lt;td nowrap="nowrap" valign="bottom" style="text-align: left; padding-bottom: 2px; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;td valign="bottom" style="padding-bottom: 2px; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;td colspan="2" valign="bottom" style="border-bottom: #000000 2px solid; vertical-align: bottom;"&gt;&lt;div style="text-align: center; font-family: Arial; color: #000000; font-size: 8pt; font-weight: bold;"&gt;2012&lt;/div&gt;&lt;/td&gt;&lt;td nowrap="nowrap" valign="bottom" style="text-align: left; padding-bottom: 2px; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;/tr&gt;&lt;tr&gt;&lt;td valign="bottom" style="background-color: #cceeff; width: 76%; vertical-align: bottom;"&gt;&lt;div style="text-align: left; font-family: Arial; color: #000000; font-size: 9pt;"&gt;Revenue&lt;/div&gt;&lt;/td&gt;&lt;td valign="bottom" style="background-color: #cceeff; width: 1%; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;td valign="bottom" style="text-align: left; background-color: #cceeff; width: 1%; vertical-align: bottom;"&gt;&lt;div style="font-family: Arial; color: #000000; font-size: 9pt;"&gt;$&lt;/div&gt;&lt;/td&gt;&lt;td valign="bottom" style="text-align: right; background-color: #cceeff; width: 9%; vertical-align: bottom;"&gt;&lt;div style="font-family: Arial; color: #000000; font-size: 9pt;"&gt;25,109&lt;/div&gt;&lt;/td&gt;&lt;td nowrap="nowrap" valign="bottom" style="text-align: left; background-color: #cceeff; width: 1%; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;td valign="bottom" style="background-color: #cceeff; width: 1%; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;td valign="bottom" style="text-align: left; background-color: #cceeff; width: 1%; vertical-align: bottom;"&gt;&lt;div style="font-family: Arial; color: #000000; font-size: 9pt;"&gt;$&lt;/div&gt;&lt;/td&gt;&lt;td valign="bottom" style="text-align: right; background-color: #cceeff; width: 9%; vertical-align: bottom;"&gt;&lt;div style="font-family: Arial; color: #000000; font-size: 9pt;"&gt;17,893&lt;/div&gt;&lt;/td&gt;&lt;td nowrap="nowrap" valign="bottom" style="text-align: left; background-color: #cceeff; width: 1%; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;/tr&gt;&lt;tr&gt;&lt;td valign="bottom" style="background-color: #ffffff; width: 76%; vertical-align: bottom;"&gt;&lt;div style="text-align: left; font-family: Arial; color: #000000; font-size: 9pt;"&gt;Percentage of Payment Solutions revenue&lt;/div&gt;&lt;/td&gt;&lt;td valign="bottom" style="background-color: #ffffff; width: 1%; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;td valign="bottom" style="text-align: left; background-color: #ffffff; width: 1%; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;td valign="bottom" style="text-align: right; background-color: #ffffff; width: 9%; vertical-align: bottom;"&gt;&lt;div style="font-family: Arial; color: #000000; font-size: 9pt;"&gt;22.5&lt;/div&gt;&lt;/td&gt;&lt;td nowrap="nowrap" valign="bottom" style="text-align: left; background-color: #ffffff; width: 1%; vertical-align: bottom;"&gt;&lt;div style="font-family: Arial; color: #000000; font-size: 9pt;"&gt;%&lt;/div&gt;&lt;/td&gt;&lt;td valign="bottom" style="background-color: #ffffff; width: 1%; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;td valign="bottom" style="text-align: left; background-color: #ffffff; width: 1%; vertical-align: bottom;"&gt;&amp;#160;&lt;/td&gt;&lt;td valign="bottom" style="text-align: right; background-color: #ffffff; width: 9%; vertical-align: bottom;"&gt;&lt;div style="font-family: Arial; color: #000000; font-size: 9pt;"&gt;16.9&lt;/div&gt;&lt;/td&gt;&lt;td nowrap="nowrap" valign="bottom" style="text-align: left; background-color: #ffffff; width: 1%; vertical-align: bottom;"&gt;&lt;div style="font-family: Arial; color: #000000; font-size: 9pt;"&gt;%&lt;/div&gt;&lt;/td&gt;&lt;/tr&gt;&lt;/table&gt;&lt;div&gt;&lt;br /&gt;&lt;/div&gt;&lt;div style="text-align: justify; font-family: Arial; color: #000000; font-size: 10pt;"&gt;&lt;font style="font-style: italic; font-family: Arial; font-size: 10pt;"&gt;&lt;u&gt;Accounts receivable, net.&lt;/u&gt;&lt;/font&gt;&lt;font style="font-style: italic; font-family: Arial; font-size: 10pt;"&gt;&amp;#160;&lt;/font&gt; We reported $3.4 million and $5.3&amp;#160;million in &lt;font style="font-style: italic; font-family: Arial; font-size: 10pt;"&gt;Accounts receivable, net&lt;/font&gt; on our Consolidated Balance Sheets as of June 30, 2013 and September 30, 2012, respectively. This item represents the&lt;font style="font-style: italic; font-family: Arial; font-size: 10pt;"&gt;&amp;#160;&lt;/font&gt;receivables from our Clients that we expect to collect. &amp;#160;Approximately 11.3% and 3.1% of the balances reported at June 30, 2013 and September 30, 2012, respectively, represent &lt;font style="font-style: italic; font-family: Arial; font-size: 10pt;"&gt;Accounts receivable, net&lt;/font&gt; that is attributable to operations that we intend to wind down during fiscal year 2013 and fiscal 2014. &amp;#160;The remainder of the &lt;font style="font-style: italic; font-family: Arial; font-size: 10pt;"&gt;Accounts receivable, net&lt;/font&gt; balance is composed of receivables from certain of our Payment Solutions Clients. None of our Clients have receivables that exceed 10% of our total receivable balance. As of June 30, 2013 and September 30, 2012, &lt;font style="font-style: italic; font-family: Arial; font-size: 10pt;"&gt;Accounts receivable, net&lt;/font&gt; included an allowance for uncollectible accounts of $0.2&amp;#160;million and $0.2&amp;#160;million, respectively, representing the balance of receivables for which we believe collectability is not reasonably assured.&lt;/div&gt;&lt;div&gt;&lt;br /&gt;&lt;/div&gt;&lt;div style="text-align: justify; font-family: Arial; color: #000000; font-size: 10pt;"&gt;&lt;font style="font-style: italic; font-family: Arial; font-size: 10pt;"&gt;&lt;u&gt;Settlements receivable&lt;/u&gt;&lt;/font&gt;&lt;font style="font-style: italic; font-family: Arial; font-size: 10pt;"&gt;&amp;#160;&lt;/font&gt; As of June 30, 2013 and September 30, 2012, we reported $20.3&amp;#160;million and $15.3&amp;#160;million, respectively, in &lt;font style="font-style: italic; font-family: Arial; font-size: 10pt;"&gt;Settlements receivable &lt;/font&gt;on our Consolidated Balance Sheets, which represents amounts due from credit or debit card companies or banks. &amp;#160;Individuals and businesses settle their obligations to our various Clients using credit or debit cards or via ACH payments. &amp;#160;We record a receivable for the amount due from the credit or debit card company or bank, and a corresponding payable to the Client. &amp;#160;Once we receive confirmation the funds have been received, we settle the obligation to the Client. See Note 8&amp;#8212;Contingencies and Commitments for information about the settlements payable to our Clients.&lt;/div&gt;&lt;div&gt;&lt;br /&gt;&lt;/div&gt;&lt;/div&gt;</NonNumbericText><FootnoteIndexer /><CurrencyCode /><CurrencySymbol /><IsIndependantCurrency>false</IsIndependantCurrency><ShowCurrencySymbol>false</ShowCurrencySymbol><DisplayDateInUSFormat>false</DisplayDateInUSFormat></Cell></Cells><ElementDataType>nonnum:textBlockItemType</ElementDataType><SimpleDataType>na</SimpleDataType><ElementDefenition>The entire disclosure for any concentrations existing at the date of the financial statements that make an entity vulnerable to a reasonably possible, near-term, severe impact. 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