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. 

Like others, with clothing items I used a ziplock bag with the air pressed out and sealed, then put them in TB2's (tough bags). 4 years of sending clothing that way resulted in 0 items damaged & 0 complaints. In fact, I found most people appreciated the cheaper postage over everything else, and it was still done carefully, nicely folded,

waterproof and all that.

Edit: I never used a straw, lol. I went down to my local Salvos and bought one of the biggest, heaviest books they had, then used that to compress the contents in the ziplock bag, then I'd seal it before removing the book. Then I'd fold over the top and tape it down securely. (Be careful of items that have delicate attachments if you do that - you don't want to whack the book down and break buttons etc  ).