Issuing Credits

[Len]

I would like to issue a 'Credit' to a customer.  The credit is not based on 
a return item nor a specific previous billed amount.   The credit will be 
more like a bonus to the customer for not returning a product.

for example:
I would like the CSR to be able to give a customer  25% of the product's 
purchase price as a credit.

I currently have an amount field, payment methods and a reason field.  Which 
entity and service would be best used  in order to store this information?