I see "gatewayid" in the client file, but I seem to be overlooking the panel where I can select the default payment gateway when I set up a client. I have four payment gateways activated.
Also, I need to set up 3 types of billing runs: clients who get mailed invoices (some don't have email addressess), clients who get emailed invoices, and clients who pay by credit card. I'm having some trouble understanding how that might work with WHMCS.