Does anyone know if I can and how much I can charge while I store items left behind by my former tenant? 

Again, the BiggerPockets store has some books for this or you can learn by watching podcasts, reading blogs, and interacting on the forum.

Not worth my time to chase them down or store their junk and if they left it behind, they clearly don't care enough about it.

Buy a vacant piece of land close by and offer to store his stuff for free until the house sells. 

Purchase price: $650,000 Cash invested: $75,000 First industrial development, project consists of 2 warehouses with a trailer drop yard for indoor storing of dry goods & refrigerated

Is there some service other stores which I can use so I have a PO Box but I get the mail forwarded to my home address.

Persons who take possession of real property (the property in question happened to be a store building) without consent of the owner are deemed to be tenants by sufferance, and will be required to pay reasonable rent for the time they actually occupied the premises.”

We have a property in Chicago, Portage Park area that we are interested in rezoning and developing into a multi-use property with 3 commercial stores on ground level and 9 residential above.  

They sell them at the big box stores.

You've got a lot of green/granola/alternative types and type stuff and stores and restaurants, and you've got a lot of downsides to that -- homelessness, anti-government/anti-vaxxers/homeopathic types.