This what I do.

Flatten clothing item and place in A4 size zip lock bag. Flatten air out and seal. Tape over tightly with packing tape. If zip lock opens bags can inflate in transit. I then use a normal manila folder. Fold over extended edge and tape edge. tape across bottom. A4 zip locked package then slides perfectly inside. Tape end shut. test it goes through card slot. Weigh, put appropriate stamps on. Post in street box. Not trackable so over counter is no advantage.

Folder is stronger than a normal envelope, and clothing dont need padding, in any case padding takes up valuable letter depth. Zip lock bag protects item in case package gets wet.. "vacuum" sealing in zip lock bag prevents item bulking up in post, and getting caught in sorting machines.