That makes sense, kind of.
My train of thought was if I get affiliates linking directly to /XXXXXX/ and that just has the index.php page in for the site, when the visitor clicks a link, it will take them to the join page where the affiliates ID will be presented along with transaction information to credit them with a sale.
The other pluses of having it redirect to https://www.domain.com/XXXXXX/ would be that i should, in theory help with search engine rankings to that specific domain name and, its a lot easier for affiliates to remember a short link such as that one, than the one provided by the payment processor to track their affiliate sales.
Ill take a look at htaccess redirect stuff and see if I can figure out the code needed to add that to the server on all of the domains where I want to implement this.