I'm thinking of purchasing the module, how do I link it to my account once purchased?
Purchasing will provide you with a key. Use the provided key on this page of foundry.com (must be logged in). Foundry has a guide for this here.
Alternatively there is a newly released official Foundry storefront where you don’t have to mess with keys at all. Has the same effect of permanently adding the module to your Foundry account.