After I get all the air out of a ziplock bag, the ones I use are too big to fit in the TB2 bags, so I leave enough room at one side and the top to fold over and tape down (so the opening is effectively sealed twice). I can't guarantee that keeps them from inflating, but I did receive one back once, after about a month (a 'Not At This Address return), and it was pretty much the same as I had posted it. 

I have also received one underpaid postage notice after using that method, so it may have inflated some while in transit, but it's the only one in 4 years and the package I got it for was borderline even with all the air out.