I've read on other forums of many members sending a fully stripped slide to Glock and having it returned assembled with new internals and RSA.
I picked up a worn police trade-in Gen 3 G21 with dead night sights a couple of months ago and sent the entire pistol into Glock for service and replacement of the sights. Glock charged my card $59 and some change and returned it with brand new night sights and internals, including the trigger assembly, takedown lever, slide stop lever, and RSA. Shipping the entire pistol can be costly, since it must be done via UPS/Fedex overnight, unless shipped from an FFL, who can send it via USPS. I elected to send it through my FFL and managed to fit it in a small USPS flat rate box, and Glock returned it in a recycled (but great condition) factory case. The only hiccup in the process was that they initially got the return shipping address wrong and listed my street address with my FFL's city and zip code, which are not the same. It took a few calls back and forth with Glock to correct the mistake, but they were able to get it fixed before it got picked up by their carrier, and all was well. Glock took about a week to service the pistol following receipt, and the overall timeline was about 2.5 weeks from the date my FFL shipped it to when he received it back. It's an excellent service that I would use again in the future if needed.