Try the rigid card mailers - there's a seller on eBay who sells them specifically to keep under the letter dimensions, and they won't be susceptible to expanding like envelopes etc are. 

I use them for more delicate items of jewellery, but I think they would be quite good for your softer items that can be folded or packaged nicely, but may bunch up etc in a padded bag or envelope.

I use rigid card mailers but more out of sheer desperation than practicality.   They work well up to a point, garments that have dome shaped buttons present a problem though!   AP applaudes.....another one bites the dust....