Hmm thanks for all your replies, I think I kind of knew already that I should just take the refund and get the jacket back.
The buyer has sent me a picture of the item since they received so they couldn't try to damage it now and say it came like that, if I did really want to dig my heels in. But I think I will refrain from trying to "win".
You could offer them a discount first? Save yourself the hassle. I know, I know, it sucks.
"Sorry about this, I genuinely believed it did have the feature you wanted. I really want to avoid the hassle of a return, so can I offer you £ off? No worries if not, just let me know".
Hmm thanks for all your replies, I think I kind of knew already that I should just take the refund and get the jacket back.
The buyer has sent me a picture of the item since they received so they couldn't try to damage it now and say it came like that, if I did really want to dig my heels in. But I think I will refrain from trying to "win".