Well, you do offer a transaction fee to the bank (if you're in the U.S. where these transfers aren't free of charge), but presumably the bank has a fixed fee sc

Well, you do offer a transaction fee to the bank (if you're in the U.S. where these transfers aren't free of charge), but presumably the bank has a fixed fee schedule for that and you don't specify a fee of your choice. A closer analogy might be a service where you can offer a tip by typing or writing a tip amount. In restaurants, if paying by credit card, you have to manually add the tip amount to the total in order to guard against mistakes in your intention (both through fraud and through your own error). Some online services like food delivery may let you specify an arbitrary tip just by typing it in, and I'm sure people make mistakes in the amount all the time (e.g. $44 instead of $4, or $80 instead of $8.00), though the web sites probably also have a limit on the tip value, so you'll usually only see mistakes of one order of magnitude rather than six.