There are a multitude of technical reasons why one shouldn't purchase glasses online. If you cannot try-on the frame before purchasing, there is a very low chance it will properly fit you. If you can try-on the frame before purchasing, it is unlikely you know how to properly assess its fit. Measuring your own Pupillary Distance (PD) is like trying to cut your own hair with garden sheers. Pupillary Distance (PD) is frame and prescription dependent. Pupillary Distance (PD), by itself, is inadequate to align the optical center of a lens with the pupil. Several height measurements are needed for multifocal/progressive lenses, yet none are requested by online vendors. This means errors will result. Online vendors almost exclusively sell polycarbonate (refractive index 1.59) lenses, which provide the worst optical performance. Moderate to high prescriptions often need higher index lenses to keep the lens thin and light. Online vendors don't typically offer high index lenses. No return policies are common among online vendors. Even when returns are permitted, shippping can be costly. Glasses purchased online rarely include a manufacturer's warranty against defects.