I recently bought a pair of cycling specs with additional clip-in frames for prescription lenses from an online retailer.
Although the correct prescription had been used, they were completely out of focus.
The local optician explained that this was because the "vertex" measurements had not been taken into account. This is the measurement of the distance of the lenses from the eyes and the position of the focus point for each lens and can only be done in a 1-1 consultation, of course.
If you're going to order, make sure you get the glasses "frames only" so that your local optician can put in the correct lenses.
It cost me an extra £38 to reolve...