Adding Bonus Items or Free items given by Suppliers

2 ways you can do this:

Solution 1:

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

Solution 2:

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

for example, if you purchase 10 quantity for a price of 200 each, 10X200 = 2000
and get 2 quantity free, then the average cost of each will become 20/12 = 1.66