Thank you for sharing your solution. As you say, there are multiple ways of solving this kind of problem. Personally I prefer to model such a relation with a third entity. Continuing your example, one invoice would have multiple invoice lines. Each line has a one-to-many relation with an inventory item. This invoice line would also contain all the extra data you need such as quantity, dateAdded etc. The invoice line table created in this situation actually takes the role of the invoices_inventory table.
I like to keep data contained inside an entity. Relations are just links between those entities.
