postal service

Postal Service Box in Chile - is it necessary for registering and buying a vehicle?

We will be in Santiago in a week and looking to get a RUT, buy a vehicle, and drive south towards Puerto Montt. In some cases I have read that you are sent off with temporary docs, and the final documents for both the RUT and for a vehicle ownership registration will have to be mailed somewhere. Does this sound correct, or can most get all of the final docs all at once?

If they need to be mailed, we will be staying near Puerto Montt - is there a system similar to PO boxes that you can purchase and use that as your address? 

Thanks! 

Carissa