>You can't ship and return these items safely.
Also it's not as though panties are as difficult as bras to purchase. T&C is trying to do the hard work; doing relatively unrelated easy work alongside that might be a distraction.
TFA is a woman and all, but it's still tempting to consider her one of those 'old boy' industry stalwarts who are so afraid of disruption from outside that they can't imagine what it would look like. If she can find one, she should ask a travel agent what he thinks of "algorithms".
We call that "add-on sales", and it's a good thing. If there were a way to do it on the site they'd probably 4x their profit instantly.