1. I do put sport coats in a box, but only because I think a buyer would think that it is improperly packed if in an envelope. I do not agree, but there is no point in fighting that perception with my buyers.
2. It doesn't cost the same. The Tyvek envelope is about 6.5 ounces lighter than the USPS 1095 box. Both ship by weight, in one pound increments. So the box will put the shipped weight over the next pound increment about 40% of the time compared to the envelope.
3. The box is easier to cut or puncture and it absorbs moisture. The perception may be that it provides more protection, but I disagree.
I might be too late to this party, but all my SC get folded into a tyvek (15.5x12?? I think) USPS envelope then slid into a Regional Rate Box A. Only way to ship sport coats IMO. Large overcoats/duffles go into Regional Rate B or Medium Flat Rate