Over the years, I’ve bought a number of cheap prescription eyeglasses made in China from a company called ZenniOptical.com. Their complete prescription eyeglasses (lenses and frames) start at $7, with an additional $5 for shipping.

Many people have let me know repeatedly on the Clark.com message boards that Zenni can be a customer service nightmare. If the glasses come as intended and the prescription is right, well, people are just happy as a clam for the cheap price. But if something goes wrong, it’s almost impossible to make it right because there’s little to no customer service after the sale.

In addition to Zenni, there are a number of other sellers of complete prescription eyeglasses for under $20. (If you wear bifocals or progressives, you can run that bill up to $40.) Some of the ones I’ve discussed in the past include EyeBuyDirect.com and GlassesShop.com.

So typically, you’ll pay one-fourth to one-sixth the usual price with the online sellers versus the traditional retailers. But again, you have the issue of limited customer service after the sale with online glasses transactions to consider.

RELATED: Prescription glasses for $7 shipped

Before ordering glasses online, get this info together

If you plan on ordering, get your vision checked by an eye doctor first to get an updated prescription.

You’ll also need to find out your pupillary distance (P.D.) — the space between the center of your pupils, expressed in millimeters. Certain frames will not work with certain P.D.s because the center of the lenses will either be too wide or too narrow. You need a fit that’s just right.

If your P.D. is already noted on your prescription, you are good to go. If it’s not noted, many doctors will provide this information upon request. If not, here is Goggles4u’s guide to calculating your pupillary distance.

Some optometrists reportedly may try to charge up to $20 when you call to ask for your P.D. Though here’s a thought: If you’ve had glasses filled at Costco in the past, give them a call and they’ll give you your P.D. for free!

I know some people can’t hang with the idea of ordering glasses online. So for everybody else, here’s another cheap route…

Costco tops tally of eyeglasses retailers

Consumer Reports recently took a close look at eyeglasses retailers, and for the first time ever, Costco Wholesale has been identified as the No. 1 place to buy glasses in America. According to the magazine, Costco’s average price for a complete pair of eyeglasses is $186, while the industry retail average is $220 to $240.

The next best place to get your glasses after Costco? Any private doctor’s office or independent eyeglasses shop. Meanwhile, a number of the big chains got horrible scores in the tally. J.C. Penney Optical, Visionworks and America’s Best Contacts & Eyeglasses were all among the big losers.

RELATED: Prescription glasses from $7 at Zenni Optical

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