I just sent off a filter starter sample, biological filter media sample, yesterday in a small flat rate box. It cost only $5.35 but it was too thin for a betta to ship in. It would damage a betta by being that thin after you squash the bag down to fit inside the box. The smallest box is only about an inch thick while it is fairly large in the other dimensions. It was no problem at all for filter media but that stuff does not need as much stable water volume as a mature betta.
Ziplock bags are completely unacceptable for shipping. Use either Cordon breather bags, if you can find them, or standard fish bags which are readily available on Aquabid. Temperature is often more limiting than anything else. In summer, at least where I live, the fish will overheat in a matter of hours if they are left outside. In winter they are going to become far too cold even in a heated delivery vehicle. That leaves only spring and fall for most shipping and really only a period of a few weeks in each season.
Unless you have uniquely special bettas, you will be trying to dump them on your LFS, not ship them to people who can source bettas locally for much less cost. If you have a unique betta, you can definitely benefit from breeding them, but who wants to pay over $50 for a single breeder as a first attempt? It is just too much cost to figure out how to do things right.
