Unwanted advice-- most old time sellers here will counsel you against partial refunds.
We tend to think of them as post-sale discounts.
If it's good enough to keep, it's good enough to pay for.
My standard Message in requests for return, replacement, or refund is:
"I regret you are unhappy with your purchase. Please return it for refund."
YMMV - but it is easy for a new seller to be taken in by unwarranted claims.