That was a rather unwise decision. Once you have proof of delivery to the GSP hub, they are responsible for any delays. You have refunded unnecessarily and buyer will still not be happy because they have paid a large amount for GSP shipping fees.
I doubt you can recover anything as you had done your bit. in delivering to the GSP.
Buyer should have opened a not received case which you would have passed to GSP to deal with. Also of course by refunding outside a case you will not get your selling fee refunded.
