Good responses already. Assuming the payee is an individual:
First, you are not required to do it. If the payee on your customer's check wanted a cashiers check, he should have demanded a cashiers check from your customer.
Second, make certain any prohibition you have in a BSA policy about selling checks to non customers indicates that it applies to "cash" sales. There is no BSA reason for refusing to accept a properly endorsed on-us check in payment for a cashiers check.
Third, if you choose to do it, you are not required to do it for free; charge your normal fee.
Fourth, if the payee is not the individual standing on the opposite side of the counter forget all of the above. This is a mechanism that some banker dreamed up when banks stopped certifiying checks. The UCC does not bless it. It is not the equivalent of certifying a check and it's a ridiculous move if the check is payable to an entity.
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In this world you must be oh so smart or oh so pleasant. Well, for years I was smart. I recommend pleasant.