Most merchant accounts use the Authorize.net gateway anyways. But does aMember require Authorize.net to be the merchant account or can I use it as a gateway only with the plugin?
at the moment I'm assuming that this is not supported. What I mean is whether I MUST create and use Authorize.net to process credit cards? Or whether I can use another much cheaper merchant to process the payments as long as that merchant uses Authorize.net's gateway.
If your merchant account is assigned to Authorize.net you can do it. That is not an aMember issue - that is done by the bank to supports the merchant account. You have to talk with the bank and have them assign the account to the gateway of your choice.