What drives the cost up for a porta potty in a hard-to-reach location?

We're hosting a weekend wedding at our property, which is at the end of a long, narrow gravel lane. The rental company quoted us for a standard portable toilet and an ADA-compliant portable toilet for about 80 guests over two days, with catering and alcohol. The ground is level grass, but the lane is too tight for their regular vacuum truck to turn around. They mentioned extra fees for 'difficult access' and possibly a smaller truck. What exactly drives up the cost in situations like this? Is it just the truck type, or are there other factors like longer service time or special equipment?