I am developing a Property Management DB system now I need to incorporate the Invoice part of it. The Db has many units each unit has many different charged like; telephone, rent, parking, water etc...

What is the best way to it?

I think this would be much like a point of sale db. Customer, Invoice ( a line item of Customer), Invoice Line ( a line item of Invoice), Price List which would serve a source for a lookup in Invoice Line.

Price List could also be thought of as an inventory. A lot easier than inventory in that you don't have to keep removing and adding products with each sale.