Hello and welcome.
I agree - 1 in 5 screen breakage is ridiculous. Sounds like he may not actually be Apple Certified. The breakage rate should be more like 1 in hundreds/thousands at the very least. You shouldn't have to bear the cost upfront of their lack of skill/training. And, if any reputable vendor did break the display glass, it's really on them and not you.
Shops do charge diagnostic fees, but normally they apply that to any repairs once a repair is deemed necessary.
I would definitely do a bit of research with other companies in your area.
C