If it was my employer then would I even be allowed to give a tip?

I mean it depends on your employer. But anecdotally, yes, business and corporate purchasers usually did tip us, although nowhere near the usual 10-20% of the price expected of smaller purchases. Actually, they would often tip in advance with a credit card payment. (Cash tips were quietly preferred but I was always thankful for any tips.)

Which then leaves an interesting idea. Why 20% tip instead of 20% more food?

Reputation. Makes your company look bad if you don’t tip at all.

If they are paying I would probably spend what ever I can to max out the limit.

I mean if you’re trying to “max out” some limit, then simply max out (limit)/(100% + (tip)%) with (tip) > 0 and give the driver (limit), and your employer should understand what you’re doing. I would fix (tip) with my employer before I set up the order. Or, tip out of your own pocket.