None that stopped me ordering from goggles4u.com.
The pair I'm wearing right now cost me €12 and so did the matching sunglasses.
Delivery takes 2-3 weeks, but other than that I've no complaint.
No, maybe I got lucky or have a very regular-shaped head — admittedly, I physically measured up my old frames and checked those against the new ones I was ordering. The goggles4u site gives precise size in mm, and if you upload a picture of yourself there's even a sort of a virtual "try them on" feature. One measurement to watch out for is the length of the arms — you want the tips of them to wrap around your ears without being too tight or too loose.No problems with the frame fit DrMoriarty? That's what I would be afraid of, the lenses would be fine, but the frames might not fit well.
Yes, if you pay to have your eyes tested they should give you a copy of the prescription, although many will not venture this, and instead tell you they'll just "keep it on file".