Mod 80 & ICOT-
I just bought a pair from both Zenni and Coastal Contacts. Both are very good. Coastal Contacts seems to run a free pair offer quite frequently. (You can find their coupon code, and when it is running, on their facebook page.) It is, like noted above, free except for approximately $15 shipping. Zenni has glasses for $7 (and up through $45) which include both the frame and standard lenses. They add $5 for shipping. So the total cost with Zenni is $12. This is Zenni’s regular pricing available all the time.
It seems to me that while Zenni makes a decent glasses (both frame and lenses), the Coastal Contacts glasses are a better quality and perhaps a greater selection of fashionable frames (including the frames eligible for the free offer.) The limitation of Coastal is that you can only use this pricing offer one time (while Zenni is always available.) Also note Zenni can take up to two weeks to receive (usually closer to one week) while Coastal is usually under one week to receive.
As far as the thinness of the lens, Zenni offers what they call a 1.57 lens while Coastal offers a standard 1.5 polycarb lens. In theory the 1.57 should be thinner, but for whatever reason it seems that my 1.5 lens from Coastal is slightly thinner than the 1.57 lens I received from Zenni.
In short, they are both very good deals, but the Zenni offer is always available for any quantity while the Coastal is a one-time deal that you need to snatch when they run their (frequent) promotions.