18F Micro-purchase gives vendors the flexibility to choose the payment processor that works best for them. All we need is a place to enter our credit card information. This is known as a vendor's Payment URL.
To specify or update your Payment URL, simply visit your profile. We'll issue payment at your Payment URL whenever you win an auction and meet its acceptance criteria before the delivery deadline. Please note that if you use a payment processor that charges us a fee, that fee will be deducted from the award.
PS: One payment processor we've seen used is Stripe. Stripe is simple to setup, and doesn't charge a fee for those issuing payment. What's more, one vendor actually wrote an open source app to deploy a Stripe payment form via Heroku.
PPS: Oh, please also let us know if you're interested in using PayPal.