Hubby used to work at a car dealership and he said he could always see 'em coming before they walked in the door: The people who had already planned their speech for something to complain about so they could get their car repair / oil change/ new tires cheaper or free. Every. Time.
You're not Walmart, where cakes are sitting on the shelf waiting to go thru the check out line. If that's how she wants to pay for it, then she needs to head to a retail store like walmart.