The Text/HTML module approach is the easiest and it should work. Are you able to verify that the user gets assigned the role after checkout?

Here is what I did.

I made the confirmation page link to appear in the menu.
I went on the confirmation page and dropped in a text/html module.
I updated the permissions of the module to be visible only to a custom role I have made which I assign to the users successfully after a specific product purchase.
I created a test user and made a test purchase.
Role assigned successfully but I could not see the html module on the confirmation page when the purchase is completed.
If then I click the "Confirmation" link in the menu, I do see the html module but of course there is no confirmation. Just a red message "Invalid transaction. This transaction has been completed or cancelled." which is normal as nobody should not be able to browse the confirmation page by hand.

Any ideas?

Thanks

This is due to DNN caching the user roles. So even though the role has been applied, the site only sees the role after a few minutes or when the user logs out and logs back in. We added cache instructions in the next release that should address this issue.

Thanks,

You are right. In order the user to be able to access any resources available to the new role we assign to him after a successful purchase, he has to logout and login again.