If the complaint is something like the scent isn’t strong enough then all you can reasonably do is send a replacement pair. One time only though.
Buyers have to be realistic. If they smoke there sense of smell won’t be as good as a non-smoker. Women smell differently, some are stronger than others. It’s a natural product and therefore there are natural variations which are outside the control of the seller.
Likewise there are some sellers who have a very light scent so they may (if they get the same comments from other buyers) have to wear the panties for a little longer than say the ‘average’ seller – whatever that is.
Weigh everything up. Are your other buyers happy? Do they say the same thing? Is this buyer a good buyer? Is he worth keeping?
It may simply be that buyer and seller aren’t suited but if you have sent a replacement then there is nothing else you can do apart from recommend another seller.