You can get away with a lot in a padded envelope, as long as you zip tie the tools to cardboard first and use a little extra tape outside.
I shipped a vintage Purox cutting torch (heavy) with extra welding tips (about 10 pounds of brass) just fine this way. It was my old reliable torch for years, but I got another one (story below).
The combination of cardboard with the envelope makes a huge advantage.
Tools shipped loose have little chance of making it.
For a while my local Post girls were keeping track of what boxes weighed.
I had second place for a med flat rate box for a while.
My buddy Jim sent me Oxweld torch stuff (the Cadillac of torches).
2 mint bodies, regular cutting attachment, twin cutting head (2 torches side by side and adjustable for distance apart), every cutting tip size made with duplicates of several common sizes, and every single welding tip size ever made for these. That's a lot.
Well over 30 pounds of brass in a med flat rate box.
I wonder what had first place in a med box??
yours Scott