What is the best way to credit an unhappy customer?

Hi,
   Occasionally we send our customers new products that they haven't seen before (we're a wholesaler who deal with the same customers again and again) and sometimes they think they're way overpriced.  Rather than doing a call tag and all that annoying jazz, I often say ok how much do you want me to take off and whatever flat amount they say is what I do.

Now I'm wondering what is the best way to account for this in NetSuite, specifically when I receive the payment and to NS it looks short, but really it's okay.

Here's an example
Customer spent $1000 receives 20% off, invoiced for $800.  Unhappy with something so takes off $50 from the invoice and sends check for $750.  When I go to credit the invoice NS wants to know what for exactly.  In the discount field it's already labeled -20% so I can't say -20% and an additional $50 off (or can I and I don't know how?, that would resolve everything!).  Basically NS in the credit memo wants too much info (enter items and manually do it that way).  I just want the ability to take off a given amount for whatever reason