Catching up on Marc Hedlund's blog today, I was reminded of a real pain I ran into a day or two before heading off-grid for a few weeks.
A credit card payment was going to come due while I couldn't get online to pay it, so I went into the issuer's web site to schedule a payment. Trouble was, the period wasn't done yet, and I knew I'd made purchases that weren't yet posted, which meant the current balance wasn't what I would need to pay to avoid finance charges. And the interface wouldn't allow me to schedule a payment for more than my current balance.
Now, I don't want to ascribe malice to this web site limitation, but it seems to me that the issuer doesn't have much by way of motivation to allow me to pay an arbitrary amount. If I can pay only my current balance, then they get to charge me interest for the whole amount for the whole period per the terms of the cardmember agreement. Were I in their boat, I may well be tempted to find a way to discourage full, on-time payments.
But I 'd only think about it.