I'd like to revive this old thread. I have a client that would like to install DT-Register, a Joomla component that processes payments through authorize.net. The client is very please with Costco processing their cards, though. Does anyone know of any resellers that are not bundling processing with the Authorize.net gateway?
Costco works on one of the larger transaction platforms - you should be able to use Costco's electronic payment gateway or just sign up for the gateway you mentioned - any reseller listed on their website will sell you the service.
You just need to make sure the gateway is specific to the platform that Costco supports.