Remeber that you pay for their time. not only the time it takes to unscrew, blow the dust away, and put back together, but also the time they use on the phone with you, the time they use taking the thing in question in, and getting it back to you.
And unless they looked at it, which as far as i udnerstand they didn't cause you would've been charged then, they cannot know, how long it'll take, it may be mroe than dust.
Dunno about this last part but it's a guess, they could ofc charge according to time, but that would leave them with lots of trouble, when there was a big problem, and they ended up spending so much time (=money) on it, that it owuld've been almost as costly to buy a new one.
So they might have a fixed price, and then amke up for the troublesome repairs with the easy cases like your dust.
And unless they looked at it, which as far as i udnerstand they didn't cause you would've been charged then, they cannot know, how long it'll take, it may be mroe than dust.
Dunno about this last part but it's a guess, they could ofc charge according to time, but that would leave them with lots of trouble, when there was a big problem, and they ended up spending so much time (=money) on it, that it owuld've been almost as costly to buy a new one.
So they might have a fixed price, and then amke up for the troublesome repairs with the easy cases like your dust.