Plain Courtesy cost nothing but lack of it in time may cost everything.
I was in our local PC World about 18 months ago, and over heard a customer saying their PC had broken, could it be fixed? After looking at it, the shop assistant laughed and said. "What do you want me to do, look at it or throw coal in it." I almost said to the customer, "Tell him to stick his advice. But I bite my tongue on moved on.
The shop closed its door at the weekend for the last time. Obviously not because of that but who knows. Little acorns etc. I have never forgot what that bloke said that day, and have never brought so much as a memory stick from them since..