There's a few different ways you can handle messages like these, and the one you pick will depend on the most desirable outcome.
If you suspect they're suggesting they'll leave a neg if they remain unsatisfied, you can simply take them at face value, put on your best customer service face and offer a resolution - in other words, are you prepared to take a hit (financially) to try and avoid a neg from a buyer, despite the circumstances being sus?
If you're not prepared to do that, then you can still remain polite and professional, and perhaps ask a few clarifying questions, and/or photo evidence (particularly if you have images of the actual skivvies the buyer was sent, so you can compare the two - I know that a lot can happen to a skivvy in 3 weeks so a photo won't prove the buyer is telling the truth, but asking them will help help show how determined they are, particularly if the claim of stains is a bluff). You don't have to offer them anything in your replies, just tell them you're concerned about it and would appreciate their assistance in determining the best course of action to take, and / or how this happened.
One thing I have learned is that a lot of the buyers who have scammy tendencies, rely a lot on sellers not knowing exactly how to respond, they rely on them not having full confidence in the condition (or quantity) of the item when it was sent, and the seller not having a great memory for items involved in transactions a couple of weeks or more ago. The best protection you can get against these types of people is to eliminate (in as much as possible) doubt - eliminate doubt about how to handle specific situations, and doubt about the condition of your items. I developed a plan for what I was prepared to do in these kinds of situations, formulated some key responses that are adapted to individual messages as needed, and eliminated doubt by photographing exactly what was sent to someone alongside the addressed package.
My replies are often instant, full of confidence, and provide very little leeway when I am 99% certain someone is lying to me. I am never accusatory, but I make it very clear I know that what they are saying they received does not match my (particularly meticulous) records, and provide an out since quite often they realise they're not in a position to attempt to challenge my facts (by providing an out, I mean I might say "is it possible you have confused me with another seller?" or something similar, or in this case I might be a bit more cheeky and say "is it possible someone else has worn or used the skivvies in the 3 weeks since receiving them, as my records show they were immaculate prior to sending?"
