Cannot try on underwear, but size is incorrect won't accept return

I could not try on the boxers as it is underwear, a three-pack.

Got home and it did not fit.

I purchased as per my size with Edgars as it seems to be the same group.

I tried to return it to Zevenwacht P&P Clothing and they said it could not be returned it as I tried it on. I went to Edgars and bought another 3-pack the same size and it fits.

How can I be held accountable for your sizing, if it is two sizes smaller in the same retail group, but marked the same size.