What is the most cost effective and efficient way to ship out of my garage?

Launched our wallet line on Kickstarter looking for 10k.....we ended up making just over 100K. With production of the wallets now underway we are looking for the best way we can ship these wallets to our backers. Here are the recommendations we have received so far.

1. Buy packaging through Uline (white bubble mailers)
2. Ship USPS first class
3. Use a thermal printer What else should we know?? I have heard of some complications shipping USPS first class internationally. Is there a way to better ensure these packages will arrive at peoples doorsteps without getting lost in customs? Any help is greatly appreciated. Thanks