I have a package on its way, and I know it'll be too big to go into my mail box.  Around my apartment complex are banks of locking mail boxes. Each bank has...3 or 4 of the large package sized slots.  For the past few days, I have been checking the mail boxes and all of the package boxes are in use.  If my package comes before a spot opens, will it just sit on the truck until there's a spot open? Or will the mailman put it on my doorstep or something?   or will I just get a notice to pick it up at the post office?  Thanks for any help!

For the past few days, I have been checking the mail boxes and all of the package boxes are in use.   

Just because they SEEM to be always in use does not mean they really are and that they will be tied up when your package comes

  If my package comes before a spot opens, will it just sit on the truck until there's a spot open?

No, No way, no how. No mail is ever left in the truck. 2 people check the vehicle every night 

Or will the mailman put it on my doorstep or something?  

that's a possibility there are many factors that influence that. If there has been any complaints of theft of packages at that complex, the weather etc etc

or will I just get a notice to pick it up at the post office? 

This is more likely IF the parcel lockers are all tied up

Thanks for any help!

You're welcome

Well, in my neighborhood, we have a centralized mail box with one package slot per 12 houses. So, only one person's packages can be delivered at a time. I believe they hold other packages until the slot is free. That's how it seems to me, anyway. I know that if I have a lot of packages coming to me, I seem to get them all in a single day (when the box is free).

I order a lot of books (like 20 per month, plus other stuff from other places). It's always seemed strange to me that (not always, but often enough that I notice it)  they get delivered in clumps.

It would appear that different PO's handle the situation differently Stephanie, you could call and ask your PO.  Here locally, UPS will leave my packages on the front porch (but that's not USPS)  In your place, I would get a salmon colored 'pick-up' slip if I was not at home when my carrier attempted delivery.  Although my porch is safe, my carrier is afraid of dogs (and mine is big) so I have no idea how it would be handled if our property was dog free.

I've seen PBS members post that their incoming packages are often scanned 'arrival at unit' several days before they are actually delivered.  Regardless of how it is 'supposed' to work, obviously some postal units have adopted slightly different methods.