Sounds like the perfect place for a scan guage or MPGuino... Being able to see MPG instantly seems invaluable to your situation.
Lightweight effective aerodynamic mods will almost always help the mpg, even at low city speeds, see:
Aerodynamic & rolling resistance, power & MPG calculator - EcoModder.com
A hybrid would probably help a lot in the driving you plan to do, though your budget would have to allow for it. It's an interesting "problem" though, you wouldn't want to give up a good tip by being late... I suppose shifting early and keeping it at full throttle until you reach your desired speed would work well enough. An efficient (if there is such a thing) CVT would be the perfect transmission for you because of the quicker acceleration (I bet just making a light really helps with the delivery time.)
Really interesting build thread, good luck!