No you shouldn't need to purchase any mailers. If you walk into your PO they should have a priority shipping stand with the priority envelopes and labels sitting out. I just grab a handful on my way out when I start to get low. (make sure you're taking the red and white priority envelopes though - those are the free ones) The ONLY time I use envelopes other than the priority ones are for international orders. You DO NOT want ship international priority mail unless you have money to burn haha.
Yes our shirts are priced at $30 because of the limited edition nature of the shirts and yes we do print on AA. We have had it happen where a design sells out and people are asking for it. That creates a sense of urgency and a REASON for people to continually check back and see whats new and what's gone. We also send out a weekly newsletter showcasing any new design as well as which ones are just about done. (Good catch on the So-Low Hoodie BTW! I need to look into that!)
The loyalty point system doesn't really cost us much because they have to spend like $7 or $10 (I have to double check) to get a point (which is worth $1). If we have to give away 1 shirt for every 30 someone buys I'm ok with that

. Actually what most people do is a save up 5 or 6 points and use it for free shipping. You can use the points anytime you want.
I built our website on Prestashop. If you're not familiar with it you can find at
PrestaShop Free Open-Source e-Commerce Software for Web 2.0. It's an open-source (Free) e-commerce platform with TONS of extra modules and plugins to expand it's ability. The loyalty point system comes standard on an install so it tracks everything for us and handles all of the points. If a customer creates an account upon checkout they can accumulate loyalty points. If they check out anonymously unfortunately there is no way to track and award points. Theoretically someone could set up an account and have all their friends order under their account just to build up points but like I said I have no problem giving a shirt away if I'm moving 30.
Def look into Prestashop though!