I'm going to disagree with the above posters. There is no reason you cannot haul dirt (or anything else that might be dirty/large/etc) in a rental pickup. And it's cheaper to use a rental pickup than a U-Haul, Lowe's, or Home Depot truck, all of which charge you by the mile. It is for this reason the rental companies all spec their pickups with bedliners (and not plain metal beds).

I frequently get pickups from my local HLE, and this particular HLE rents out its pickups to contractors and construction companies all the time. On many occasions when I go to pick up (no pun intended) there are woodchips or other mulch scraps in the beds.

It probably would be a good idea to hose it out and vacuum inside if you really mess it up. I would be extra careful with shovels/rakes etc because dents and scratches ARE the kind of damage you can get in trouble for.

I also wouldn't recommend using the truck for towing anything (does not sound like you will), which is expressly forbidden by the rental agreement. Putting stuff in the bed and hauling is not.