I’m trying to let our members create and purchase a single mailbox on our existing domain. So far I’ve cobbled together a custom form and automation workflow, but syncing passwords between our site & SmarterMail server, and provisioning the account cleanly is still giving me trouble.

Has anyone used WHMCS + Smartermail module for something like this? Based off of what I've read it seems the WHMCS/Smartermail module is more about selling the domains with mailboxes.