This is very confusing.

You say you need to spend $1500.00 in order to get the 10% credit. You claim you spent $1700.00 so why did not receive the credit if you spent your $1700.00 like you claim.

Next instead of fighting for the $200 you are fighting for $20.00. This does not make any sense to me at all. I think you should try re-writing the letter.