You know, as one of those low-magic-campaign-runners, it's not that common IME for adventurers to have mules with them – on the initial expedition. Deserts they stick to horses or camels, jungles they've usually got or hired a guy good enough to hunt for the group. A mountainous trip maybe might call for a mule or three. Now, after they've secured the site and come across the big heavy treasures they can't pick up themselves, that's where the mules come in, on the return trip.