There's nothing with these quasi-craftsmen, even God crosses himself at them. I'll install the Velux windows myself too, since Velux installers who do some flat-rate course are anything but real masters. One guy would charge me extra for cutting the corrugated roof sheet edge, which I do myself in 5 min after measurements; another doesn't bother because as a carpenter he has way too much work; the third complicates right from the start, saying it's some tough procedure.
At home I have all the tools: saw for cutting rafters, planes, table saw, elec. screwdriver, level, so with installing four windows on my own, I save gratis another window. The warranty on the windows every buyer has, only if you don't install it well, you repair at your own expense.