I wouldn't, personally, recommend paying BEFOREHAND, however if you are comfortable in that action, that is on you.
I haven't seen many tips to the BUSINESS, as a whole.. I have, however, seen a small tip to the INSTALLER (for a job well done).
Check out your car after you get it done.. Look at the very edges of each glass (rear glass, around defroster, or third brake, etc) look for any tearing, or pinholes of light. bubbles are okay.. they will go away.
<---installed tint to my own car
<---have payed for tint
<---used to install tint in a shop.
A tip of $20.00 would be MORE than enough, if you did decide to tip the installer.
PS. GO FOR THE 20%.. you won't notice that much of a difference on the OUTSIDE, with the color of your interior, but the light difference on the inside will be WORLDS worth it.
Ray