Adding Bonus Items or Free items given by Suppliers

2 ways you can do this:

Solution 1:

When adding purchases simply make the purchase price 0 for the free items.

Solution 2:

Find the average price of the item and use that for the unit price.

For example, if you purchase 10 quantities for a price of $2 each, 10X2 = $20
and get 2 quantities free, then the average cost of each will become 20/12 = $1.66