I understand the issue. The issue is that PaymentsHttp is sort of a wrapper around the standard Http class which uses the named credential (merchantcredential) associated with the payment gateway adapter (apexadapter.name) for the endpoint (it prefixes the httprequest.endpoint uri with the named cred uri upon http send) Unfortunately there is not much flexibility here.
The doc is (or at least used to be) pretty weak on this.
The suggestion is to build the request assuming it's using this named cred uri as the endpoint. Happy SFCC :)
SELECT Id, PaymentGatewayName, PaymentGatewayProvider.ApexAdapter.Name, MerchantCredential.DeveloperName,MerchantCreditial.Endpoint FROM PaymentGateway
String additionalSuffix = "/whateverhere";
String ep = additionalSuffix;
if debugging upon http send the endpoint ends up being