If you guys keep getting backed up or too much traffic just keep raising the rates to extreme to slow people down. I think that's better then just shutting things down, right? Maybe that's a little too drastic.
Maybe something more convenient for us and profitable for them would be a bidding system. So have the system say "we process transactions 4 times per day, and while we try to get to everyone, if there are not enough available funds at the close of the next transaction processing time, those that have bid the highest percentage fee will receive first priority". Or, having the rate rise automatically as the available funds go down (as you said) is also a good option. Everyone agrees that this service is completely awesome and reliable; if I needed funds on that day, I'd certainly pay a premium to cash in through the site since I know it will be hassle free.