Thanks, everyone! We've tried Quantum but it's using our order form to transmit the credit card data which means that we are still on the hook for PCI compliance. They need to have no CC data stored or transmitted. I like authorize.net as well, but the WHMCS API for it has the same problem that Quantum's has - they both use the WHMCS forms to transmit CC data. Also, there's a weird error we're seeing with customers who had used the Quantum Vault and switched over to Authorize.net CIM; the system won't recognize it as a new entry to the CIM and so it tries to modify existing credit card info instead of creating a new user. End result: the transaction fails, every time.
I'll look at 2checkout.
Thanks again!!