I was thinking, I suppose, of Google AdWords, which has a pre-pay model-- you pay a chunk of money (as much as you want, with a set minimum), and they let your pre-paid funds are getting close to running out, so you can add more.
I was thinking, I suppose, of Google AdWords, which has a pre-pay model-- you pay a chunk of money (as much as you want, with a set minimum), and they let your pre-paid funds are getting close to running out, so you can add more.
Other usage patterns might assume regular, monthly usage.
Since I just launched this and have only a handful of users, I'm yet to see how it'll be used.
Edit: I misread your comment. They will be using this, in the best case scenario, regularly on ongoing basis - so not "monthly" in a sense of regular subscription services.
So, from my point of view as a potential user, I'd be much more likely to just pre-pay a fixed amount.
Also, I just got my first paid monthly subscription users this morning (1st day of launch, woot!), so I might be keeping this payment option for now.
However, I try to avoid recurring charges as much as I can myself, so great pre-paid options are a must.