That flat rate box is very, very small--DVD size. These pants will not fit in that box. The USPS Priority Box that fits pants costs $10.95 to mail. The USPS deliberately did this so that they can make lots more money when people ship clothes, blue jeans, t-shirts, etc. The cheapest cost to ship a pair of pants is USPS Parcel Post. I have found this out by shipping over 1,000 pair of pants all different ways.
I thought that you were talking about the really little box--not an envelope. I do not ever send things in the soft flat rate envelope; however, I could send these to you that way if you are the winning bidder. If you win the auction please let me know how you would like these pants sent to you by USPS mail.