I wish every driver would photocopy all the revelant clauses in the National Master Contract and their regional Supplement and hand them to their Helper on Day One. It isn't about what the Helper thinks, it what's in the Contract.
I suppose it's asking too much, but it would also be nice if the appropriate federal DOT, state, and local laws and ordinances were provided as well.
Typically, what is permitted locally is ok until something goes wrong. Let a bicycle cart or golf cart cause an accident, let a high value package be stolen, let a Helper get mauled by a dog, let a Helper cause any kind of an incident on a homeowner's property, and suddenly the Town Fathers realize it's costing the Town money, for police, ambulance, courts etc. This ticks them off when they realize UPS is making money on the operation. Anytime something is done commercially, it is eventually regulated, even if the same act done non-commercially isn't.
Usually it takes an ugly incident to get the Town Fathers to act, so UPS may, in some areas, fly under the radar screen for a season or two. It may also take a while for someone to report a violation of an existing ordinance.
It may be illegal to drive a cart on a public street, or on a sidewalk. It may be illegal to block a sidewalk with a cart. Permits may be required. Storing packages in a drop-off location may be a violation of zoning laws. There are all sorts of issues to consider, and they vary from place to place.