If you are looking to have a store integrated into a shopping cart, I might be able to help. There is a shopping cart called CS-Cart. Do a google search and it will be the first one. I can sell you a module that will sync all the user data to whmcs. You would then edit your template to point back the CS-Cart for changing the password and user information.
Basically, when the user signed up, or changes any of the information in CS-Cart, it sends that data along to WHMCS via the API. I mainly use WHMCS as a business portal where I can charge business and create quotes. I rarely use it for the hosting side of things. But it works really well for me. I am not sure your situation, but you need to make sure that you need them integrated. If you are just using WHMCS's cart, then there is no reason to have the two integrated. You just need to have the templates changed to match the site and logins pointed to WHMCS.
I use CS-Cart as the main user repository and this allows me to authenticate WHMCS, Kayako and recently Invision Power Board to the main site.
From the sounds of it, you need to make sure of what you want before you spend any money. There be some deep rabbit holes in these areas.