Use Paypal & E-junkie for your first few months and sell folks lifetime subscriptions to your service. $5 a month for e-junkie (not strictly necessary but it will save you a few hours of integration work), ~3% for Paypal.

After you've got a better handle on your market, have traction, and have passionate paying customers singing your praises, switch to a subscription billing provider and grandfather in the old users at free for life.

This assumes that you have nearly zero marginal cost per account, or a growth curve which can absorb the early adopters as a cost of doing business.