As @Nesrie pointed out, for me at least they are pretty much an out of pocket only kind of operation. I use them because after some disputes with my insurance company around their durable medical equipment coverage and a very large balance billing I decided it was easier and less stressful to handle it myself with cash than bother with the complete racket that United Healthcare offers. If it costs me more in the end to do it this way I just consider it the cost of convenience and preservation of sanity. :-)
YMMV of course. FSA/HSA might be a good option.