Payment Processor Recommendations for SaaS / API Service

I am starting a site that charges customers for the use of an API, Web UI Components, and some other things. There is a monthly base price, and then there is monthly usage fees.

Is there a payment provider out there that:

* Is easy to setup * doesn't require merchant accounts * handles recurring billing AND usage billing * only charges a fee on transactions, not an additional monthly fee?

Also, is there anything I should watch out for when evaluating the (hundreds?) of payment processors out there?

Some sites I've looked at so far (though, by no means do I prefer any of these): Stripe, FatMerchant, Chargebee.

I've been a user of Freshbooks for years with my software consulting business. Does anyone just use Freshbooks for SaaS billing?