Calculate item costs

Hi there,

this is my first post, cause I really need your help out there.

I am trying to put together a sheet to calculate item cost after shipping and taxes. To be honest I may have complicated the whole thing but what I have is two

order volumes that need to be changed to calculate the most cost effective solution.
For example: Item 1 (a1) is $40 and item 2 (a2) is $10, if I was to order 100 (a3) of (a1) and 50 (a4) of (a2), then multiply them in (a5) and (a6) to give me total

order values without taxes etc. I then have (a7) which is the shipping (a8) the import tax and (a9) the VAT.
What I am stuck on is how to work out how much a1 and a2 costs me per item, if I was to order 100 of each then I think it would be simpler by calculating

the total cost and divide by 100 then again by 25% which is the cost difference between $40 and $10. So although I may have gone around the houses on this

one, I want to be able to change cells a3 and a4 and then see how much it is costing per item after shipping and taxes.
Thanks for looking.