Why is the delivery fee higher for a single porta potty?

The delivery fee is higher for a single portable toilet because the rental company incurs the same fixed costs to send a truck and driver as it would for a larger delivery. The driver's time, the truck's fuel, and the vehicle maintenance are costs that do not scale down for one unit. The company sends the same truck and driver whether they are delivering one unit or a full load. The delivery fee covers the trip from the yard to your site and back, which is why it can be comparable to the weekly rental rate for a single unit.

The delivery fee per unit does get lower when you order more units. Spreading the fixed delivery cost over multiple units reduces the fee each one carries. The fee is also influenced by the delivery distance from the rental company's depot and any special site conditions that require extra time, though you mention your parents' gravel driveway with a turnaround offers good truck access.

For your wedding with 140 guests, the standard industry guideline suggests more than one unit for a multi-hour event, especially if alcohol is served or the crowd has many women. Adding a second standard unit or considering a deluxe unit with a sink might improve guest comfort and could make the delivery fee a smaller portion of the total cost per unit.