Medicare & Medicaid: what pays for check up and what pays for glasses and rims?

My mom just had her eyes checked. Medicare paid for the exam. They provide a small amount for lenses and rims. But here's the catch: We went to one of those national "mall stores" to get her prescription filled. They had a buy one get one free special...they would not accept her Medicare payment AND let her use the special. Her glasses would have come to more than $300.00!!!
So I sent her Rx off to the place where I get my glasses (it's called 39dollarglasses.com. They filled her Rx and shipped it within days. I like the quality - although styles are a bit limited.) She bought a pair of reading and distance glasses for much less than the local stores charged.