Turns out my wife read my email and already sent the payment to GPD as a gift for me. I don't know whether to be happy about it or not.
Anyway I sent them this message to see if they might partially refund me or something:
"Thank you, although I must say I am disappointed you charged me so much for something which should have been covered under warranty.
I purchased a GPD XD and a GPD Win 1 before backing the GPD Win 2 Indiegogo campaign, I may not be your best customer but I have spent a lot of money on your products.
This faulty battery is the only problem I have had, and it's happened in under 6 months of receiving the GPD Win 2. I understand that you are a small company and therefore cannot provide the kind of service that a much larger company can, but I am still disappointed you charge $65 for something that wasn't my fault.
Hopefully the new battery works fine. Thank you for the quick response anyway."