But when you go in to pay, the price after sales tax ends up being something like $57.63 (or whatever, you get the idea). So for checks, not an issue, but for cash... they now have to keep loads of coins on hand, lots of small bills, and presumably reload those often.
Either raise or lower your price. Someone buying a $57.63 cake isn't going to balk at paying an extra $2.37. And if you make it $60 even, after tax, you don't have to keep anything but $10 and $20 bills in stock for your best-selling cake, because you'll get a mix of bills in, but the only way you give change will be from a $100 bill (2 x $20) or a $50 and a $20 (1 x $10). You will still get other bills in sometimes, and some coins, and of course they do sell other sizes (but they don't do a lot of small-scale things like cupcakes). Most sales require an order in advance.
Just cutting down on all that cash handling would be advantageous (and that's why one of the owners is always at the register).