Seems like there's a simple solution to this:
I have a calendar item that pops up every month and reminds me to check my balance.
When it gets low, I recharge it with enough to last me for anywhere from 3-12 months at a whack because sometimes I'm busy when the reminder pops up.
It's worked pretty well for a long time now...and when it didn't, I have a reasonably configured mail system and I get several messages from Colin reminding me that I forgot for several months in a row. Failing all that, I'm pretty sure tarsnap will start failing backups, and if I'm a competent geek, I'm monitoring cron output for failures...especially when it deals with the ability to recover data in a disaster.
If there was a way to store a card and recharge based on settings (maybe "When my account balance drops below $x, automatically charge $y to the card") I would probably use it...but I couldn't care less if it got implemented or not.
-A