Integrating a payment gateway

Hi all,

I have a site that needs to sell a single product (a virtual currency). Payments will come from a single gateway (Moneris), although addition gateways may be required in the future.

Making use of eCommerce or Ubercart seems like overkill, as I have no need for a shopping cart or inventory. Gut instinct tells me that building it completely custom is a mistake, especially since I future requirement may be additional payment gateways.

Has anyone done something similar before? Should I be looking to eCommerce or Ubercart, if only to take advantage of the payment gateway abstraction? Is something like pgapi.module a better fit?

Thanks!