Zenni Optical - US$5 shipping (usually US$9.95 flat rate) for purchases over US$50
Zenni Optical offers really cheap prescription glasses, and I have purchased from them numerous times. The items are shipped from Hong Kong.
I have been very happy with their service.
Especially VERY happy with my sunglasses (over a year old now) -- and I wear them daily!
It is rare for them to offer any sort of discount, so $4.95 OFF their flat rate shipping is quite good, especially seeing as you can get prescription glasses from $6.95 (before shipping)!