Greetings fellow TSP shooters!

I have purchased several large orders from BulkAmmo, and am looking for the best way to calculate the price per round when factoring in the cost of shipping.

For example: lets say I recently purchased 1000 rounds of .45, 5000 rounds of .22, and 2000 rounds of .223. Subtotal was $X and shipping for the order was $Y for a total amount plus tax of $Z

Currently I add up the total number of rounds purchased and divide that by the shipping cost (8000/$Y) to get shipping per round. But this doesn't seem to account for different bullet weight within the order. Shipping a .45 should cost more than shipping a .22 round, right?

Any help or thoughts would be appreciated.