I’ve not had that happen with any that have gone out…But i kept a pair in a bag without any air at all for two weeks without opening (as the guy cancelled his order and I just forgot about it!
It DID go mouldy, but thinking about it i’d also done a pussy pop, and probably got sugar inside the little pantie ziplock bag. There is nothing in pussy juice that would make it mould like that, certainly not in the time frame. So it wont be your cum going mouldy
Most of the guys that buy from me keep their panties in the ziplock to keep it fresh, and i’ve never had a report of mould. Though think back to when you put it in the ziplock, had you been eating something sugary?
I’d say if he has a good rep then take him at his word on this occasion. I’d send a new pair rather than a refund unless he ends up asking for one. He wanted your panties, and i’m sure would be happier recieving them in perfect musky condition.