You only really need to send stuff to "technical" repairers if waterproof is 100% a requirement. If you aren't going on an expedition, or wearing a dry suit, well...considering that most membrane-style wear doesn't actually work to keep you DRY, this is a low requirement for most repair work. For patch work, it's usually enough to clean the garment (properly), patch, then follow up with a DWR on the patch area. For hemming/sleeve shortening, I wouldn't even worry about it. Ask them to use waterproof seam tape if you are concerned.
For what it's worth, the local seamstress [which I suppose I should include "tailor" in this, since it's 50-50 around here] charged $40 for a zipper replacement, and most of that was because I specifically asked for a Riri zipper that they needed to order. Finish work was excellent, and it added 5 years to the life of the jacket...humorously enough, the parts of the jacket that are failing are the parts that were ultrasonically welded. That's right--the part that "would never come undone" is the part that's failing, not the stitching.