@twisted1919, But when you say "Become the next Email Service Provider" on CodeCanyon then your system have to work on credits. Otherwise there will be a big issue for example my experience last year with MailChimp some of the emails were got stuck, but the purchased credits were deducted. So they added the campaign 34235 credits back to my account.
Many times in Email Service Provider's life we've to ADD or DEDUCT random number of credits to user account.
I'm waiting for your reply. As you're the master of this application and guide us better.