I'm planning on shipping my shirts first class via USPS. I'll be shipping single tees using the 9 x 12 white poly mailers (http://stores.ebay.com/VALUEMAILERS...?_fsub=11&_sid=26847069&_trksid=p4634.c0.m322). What I am confused about is do I need to be putting these envelopes in a first class envelope or is this all I need with first class shipping? Thanks.