Being a negative, the delivery driver arrives and demands you give him a pizza.

The employee will ignore the negative sign in front of the number and make you that pizza anyway. The calculation of the total is where it might give you free pizza.

Yeah that's what I figured. The wording of the post made it sound like it had something to do with the bacon though. You could just do this for any arbitrary pizza.

The bacon is there so you pay something, a free pizza would be weird even to a minimal wage employee (not that they're dumb, but they probably don't care).

Though that's assuming the people making online orders even bother looking at the amount paid. I don't necessarily see why they would (or why they wouldn't assume you had merely cashed in some sort of promo or balance).