Having your own large quantity of them really makes a big difference when it comes to segregating varieties of seed and or distributing seed or chili powder.
For seeds the 2x2 should be fine. I use 1.5x2 2 mil that I get off Amazon. I would think the 2x3 4 mil would be good for things like sending powders. I personally hate the paper envelopes. Whenever I get seeds in them I replace them with plastic baggies. Easier to reseal.