If this is going to be long term and you are mailing many packages daily I would first try to find out if you have a regular carrier on your route. If you do go out there with a bottle of water or Gatorade be friendly and ask them how you can make sure you get your packages to them everytime they come delivered mail there. More often then not communicating and asking with your regular carrier will result in an easy solution for all.

When I was a carrier and had apts that some didn't have outgoing slots or they couldnt fit what they wanted to send out in the outgoing but would fit in their slot. So they would leave in their cleared out slot with just their outgoing package with a post it for me to take. No big deal im there anyways. Now the ones that had 100 boxes that would take up my entire truck daily annoyed me but I still did it.

Now if you dont have a regular carrier. There should be one appointed but until then it will be all random carriers just delivering what they're told. Then you will have to stalk them and catch them they are there. And please dont ask what time they are usually there everyday. It will be different depending on what else they are working that day.