You can also just upload the design to cafepress (http://CafePress.com) and then link it on the blog. Then anyone surfing the web can go and order one, and you get a cut. You don't have to process any order, the money just accumulates in your account until it's enough to be worth a check.
Not sure why there would be such a difference. The actual shirt was the same from both Zazzle and Cafe Press. The color of the design from Cafe Press was way better from that of the other two.