In an effort to appear unbiased, many people use they in place of he and she and their instead of his and her. This is grammatically incorrect, because subjects and pronouns must agree in number.
The easiest solution involves changing your subject to its plural form. Then it’s acceptable to use they or their. For example:
The customer must include his receipt with his return.
Customers must include their receipts with their returns.
Or, rewrite the sentence, and eliminate the pronouns entirely:
All returns must include a receipt.
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