You will need to check with both state and local health departments to find out what regulations they have in place regarding pizza/pizza by the slice. As a general rule, you will need to hold the pizzas at a minimum of 140F, but if it is allowed to drop below 140F, you may be required to reheat it to a minimum of 160F (this actually means 165F) before serving. In some places the delivery time for pizza is limited to that in which the pizza can be maintained at a minimum of 140F (which actually means 145F). In some places they recognize what is called the 4-hour rule (product may be allowed to remain at a temperature which will support bacterial growth for a total accumulated time not to exceed 4-hours. Unless each pizza has a tag on it, this can be difficult to document, hence the reason why some places don't follow this rule. I believe the 4-hour rule is why New York pizzerias (slice shops) can get away with allowing the slices to sit in a display case without heat while waiting to be sold. Again, be sure to check with state and local health departments for the rules governing your location.
Tom Lehmann/The Dough Doctor