It is difficult to make sweeping generalizations about any optical store,
because they carry products of varying quality and have many different
stores each with their own staff members.

However, unlike chains such as LensCrafters, Pearle, Penny's, EyeMasters,
etc, both Costco and Walmart/Sams do carry at least some high quality brand
name products (instead of "house branded" lenses found in the other chains).
If you ask the staff, you usually can find out exactly which lens model and
which lens material you are getting. If they don't answer to your
satisfaction, ask someone else or ask to speak to the lead optician.

For whatever you get at Costco or Walmart/Sams, you are not likely to find
the exact same product much cheaper elsewhere.

Quality of fitting at Costco and Walmart/Sams (very important, especially
for progressive or aspheric lenses) varies widely, but is usually as good or
better than other chain stores. But again, working with the lead optician in
any optical store is usually your best bet.