Complaining that someone else has an obligation to tell you how to abide by simple rules is a poor argument in the first place. It is worse to then use the argument that multiple emails are "fine print" because one prefers 24 point to 8 point font.
The only logical conclusion here is that businesses create more problems by sending these notices than they solve. Best not to warn at all. Put it in the t&c, program the software to delete credit balances after the expiration date and be done with it.