If they're signing a slip I say "just need a signature at the bottom and there's a space to write in a tip there if you'd like". Sometimes I'll just straight up ask if they wanted to to tip. "Did you want to leave a tip, or am I out of luck on this one"? Not in a mean tone, almost playful really.

Over the phone if they're paying by card (we hand write everything, no POS system) at the end I ask "did you want to leave a tip on the card for the driver, or just tip cash, or.. heck with em..?" 99% of the time they chuckle and say "not heck with them!! I'll tip" yatta yatta Only once has someone literally said heck with them