This doesn't solve your coupon code and shipping question.

But I wanted to let you know there is a credit system in AbanteCart since version  1.1.5, here's the doc info,  and it works really flawlessly in 1.2.7  with the customer able to apply the credit to their order.   Prior to 1.2.7  if the total went to zero and the payment selection was still required.  If it was more than zero all was good.
 

Perhaps this will help you somewhat. 
And hopefully I or some one else will have a specific answer for your original post. 

BTW  what version are you using?

Lee