Refund amounts and timelines

[my.chic.boutique]

http://www.ebay.com/itm/2-Yds-x-45-Chintz-Fabric-Forest-Green-White-Floral-Bouquets-Cotton-Material-...

Someone bought all 14 yards and rather than wait for an invoice, they paid $27 in shipping.

I was able to ship it in a regional A box for $10 (numbers all rounded off). I've refunded the difference, but how can I get credit for that on my fees account?

I called eBay and got a foreigner who just could not understand my question. Kept telling me I can void the shipping label. Well, oh duh- I'm shipping the package today. That's not the problem. I hung up.