You can tip a UPS driver if you want too although they don't recommend cash but instead just regular gifts but you can give the UPS driver a cash tip if you want too and it's up to them if they want to accept it or not.
I have given my UPS driver a $20.00 tip before because he has done a great job at delivering my packages safely and has never damaged any packages I have brought to my house through them.
I don't normally tip a UPS driver but he's been really nice and handles my packages safely so I decided to tip him one day near Christmas and he was very grateful for it.
You don't have to tip the UPS driver but you can if you want too and I'm sure they'll appreciate it.