They specified an item for you to purchase and they would reimburse you up to $74 for that item? Was that specific item $74 or more? Or is your purchase well over a hundred because you bought several things?
Do you have the email or shop instructions that indicated they would reimburse up to $74? If indeed you only see back the $55.75 when the check comes, contact them. It is also possible that the email notice of the $55.75 is in error.