You mean other than getting what you pay for?

If you're making a simple pair of single-vision glasses, or duplicating a
prescription you already wore, it's easier to get satisfaction. If you're
making your first progressive, or strong add powers, or wildly different
prescriptions, you're better off finding a local optician. Every internet
optical advertises some sort of "guarantee" but they likely won't remake
glasses unless you convince them it's "their fault." If you have problems,
it would be fair to offer a local optician $20 to read the Rx and PD from
the glasses, so you can tell the lab what they did wrong.

Simple, single-vision lenses are made from "finished" blanks with the front
and back surfaces factory-made, with factory coatings, very reliable. The
problems arise from edging and handling - scratches, gaps at the frame,
off-axis, or centered wrong. Or right-and-left reversed - that happens a
lot.

More complex prescriptions are made from "semi-finished" blanks where the
back surfaces are ground to order by the local lab, who might also apply
coatings. The extra handling introduces surface defects, waves, coating
peel, and prescription errors.

That said, I've seen a couple of remarkable high-minus jobs made in
Singapore. One was glass, with a 1-mm center thickness. Made for a -900
billiard player, against all advice.

-MT

Dear Dicky,

Here is a source of glasses -- that
are low cost, about $20

http://zennioptical.com/cart/home.php

That way when you "break" them, you
can replace them.

I understand that the quality is OK.