Huh, how do they get that option? There have been times when I'd be totally willing to pay 20 or 50 GP to a bunch of bandits, to avoid having to encumber myself with yet more loot before I could get back to the shops...
Nah - for what the carters charge, I don't know how they can even pay for their own carts, let alone protection money to bandits on top. If I were in their business, I'd be charging something in 3 figures, and I wouldn't set off until I had at least three paying passengers. Then maybe I could afford protection.
Maybe - just a thought, it's another thing I haven't tested - the carters are all, like, 80th level, and the bandits know it?