I've not used this technique for shipping cookies, but for putting in/taking out air from ziplock bags. I seal almost to the end and insert a straw then exhale or inhale depending on what I want to do and pull straw out with my lips while closing the bag. Works great.
I don't normally use ziploks for shipping, just in a pinch if I need just a little extra cushion & I'm out.
Using a straw - brilliant! Thanks for the tip!