Stores may package the account with other services or just sell it outright. The biggest question is in whom services the account. One word of caution, if the company is reselling your accounts, with your name, you need to make sure they are representing you correctly and not underselling you. Commission and referral structures for resellers vary greatly depending on the area. In the US, ISPs usually pay companies for every sign-up they get from that company.
Payment can be up front or a recurring amount.
Paying on a recurring basis keeps the company involved, but is often an accounting nightmare. This varies on the service they are referring. Tracking the results of your programs is very important. These results are very important to understanding the effectiveness of your programs and actually paying the referral fees. Make sure that your billing system has the capability and that you can actually give your users credits or cut checks.