First off, is there any way you can get to the mail before your Mom does?
If so, do so. If not and you end up having to order another one, I'd go with either GC3B again, or Underworks. They both can help you with sizing, so you get one that fits properly. And IIRC, both will take a non-fitting binder back and exchange it for one that fits should there be any problem. Make sure you do the research as to how to safely wear a binder, you don't want to end up with broken ribs or worse!
It sucks that your Mom is giving you a difficult time. Have you come out to her yet? If she's not accepting of your situation, I have no idea how old you are, but have you tried going to a LGBTQIA center and getting yourself in a support group? Its a great way to deal with problems that family may give you. You should only have to deal with these problems until you are out living on your own.
If you have a friend you trust, have your items shipped to their house. Its a shame that you have to do this, but if it means you can he happier, then its worth the hassle.
There are a LOT of videos on youtube to help your Mom understand that "this is NOT a phase." Hopefully your Mom will come around.
Good luck bro. We're here for you.
Ryuichi
