I pay the postage with my credit card, I'm not sure if they have other options. You have to keep a balance with them, but I don't think there is a minimum balance. You can add postage in any amount. The whole process of adding postage takes about 2 minutes, so it is no trouble to essentially pay as you use it.
You can do customs forms online, there is a link that takes you right to them.
I've been using it for 10 days, and I'm hooked. Some other awesome features... Delivery confirmation is free on Priority shipping, they will email the customer with their tracking number, they will email you each day to tell you which packages were delievered, you can arrange for carrier pickup through the site and it is very easy, they check each address for errors, you can copy and paste the address from your paypal emails right into the address box...
The only negative is that you have to pay for a whole year up front. (Well, that may not be true for all levels of membership.) But I have no reservations about that after using the free trial.