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Users won't think of the cost every day but they'll notice different monthly charges on their credit card statement and wonder how they can save money then. Worse for B2B when changing amounts raise questions from other departments and need to be re-approved or justified.

How about plans that cover 15 transactions, 30, 100 and users can take over any unused transactions to the next month? I've seen services where you pre-pay packages of 100 or 500 (text message sending, Skype minutes) even which will lead to irregular payments on your end.

I think for tax purposes you need to set aside income from unused transactions. That's why often in the fine-print it says they can expire or loose value over time.

Personally I'd do monthly plans though. Some users will overpay or even forget they signed up. In that case offer generous free refunds or even email them if they haven't used the service a certain amount of days/months and offer to cancel for them.