Ask HN: Best payment gateway for storing credit cards?

I'm needing to implement a pay as you go service where you'll put in $5 and that amount gets used up over time as you use the service. Then the user can have an option to auto-refill when they run out.

With this, I'd need a payment gateway that supports storing credit card info and billing at irregular times. Gateways like Authorize.Net can support recurring billing, but it is only at fixed intervals like a day, week, month, etc.

I'd also like a payment gateway that has low monthly minimums. This isn't going to be doing a lot of volume.

Some options I've seen...

* Authorize.Net's CIM service... haven't seen pricing on it, probably varies with each reseller.

* Braintree's Vault... comes highly recommended, but last I heard they still have a minimum of $200 in monthly transaction fees.

* TrustCommerce's Citadel service... read it is $10/month with $0.10 per stored card per month. With a "pay and refill" type service, that could get costly if the user uses the service once then disappears. Would perhaps remove old card after 6 months.

Anyone know of any others?