The reliable online store should provide the detail information of products, such as the description of the eyeglasses, the photo, delivery time and shipping cost. Then you shoul take the price into consideration. The same eyeglasses may be different in different stores. You can compare the price and choose the reasonable one which can help you save much money.
You may goggle "cheap eyeglasses"online,then you will see many online stores, usually the result ranking in the front are the most popular and famous one, you can also go to their facebook wall, twitter or their website to see customers review, you may learn a lot from there. Compare the price/ quality and the styles of all products. Ask your friends who have purchased eyeglasses online before, the recommendation from them will be much appreciated. Hope you can have a nice purchasing experience.