Add/Deduct Email Credits in User Account

You can't. Mailwizz users system does not work on credits, it works on groups that have permissions and limits.
@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.
If you have such situations, you can as well move the customer in a group that compensate the loss of the credits or you can simply reset the customer sending quota. That's all we have for now.