Honestly, they do such high volume in the lens lab when a frame that isn't from Costco is sent in it slows down the process dramatically just because they don't already have the pattern for that lens to trace. There are other things that can cause issues with the frame as well that the lab may deny. By the time I left Costco I was seeing 70% of the ones we sent in to see if the lab would accept, were denied.

If they're plastic, a lot of times ther is also concerns for the lip that holds them as it fades over time when pulling and pushing lenses in and out of the frame. Sometimes the lab has to do that more than several times to get the best fit and if there are concerns for possible lens not fitting or breaking the frame they won't take it for those reasons.

Honestly though of the companies I've worked for, I still shop and get my glasses from there. The only thing I was ever told to upsell was multiple pairs since they make more money on volume. Never had an issue trying to use an outside frame if we could.