VB alternative to if function?

I use a spreadsheet to keep track of stock at work.
The invoices are put into rows with the stock code and qty next to each other.

IE:
Invoice No | Stock Code | Qty | Stock Code | Qty | Stock Code | Qty | Stock Code | Qty ect..

I am currently using an If function to check all the cells for an instance of each stock item in a total of four columns.

=IF($P8=AH$6,$Q8,(IF($R8=AH$6,$S8,(IF($T8=AH$6,$U8,(IF($V8=AH$6,$W8,0)))))))

We have now found after a year that four columns is too little for what now need. We need more like 12 columns.

The workbook is now at about 8 megs because I need to have this for each stock item and for each row on every day (66 rows by 55 columns). I run one sheet for each day of the month.
It works fine, and I will do it that way, it just opens such a margin for error and makes changing something real work, besides the really long tine it takes to save..

Would the best way forward be just to use the IF function and just embed another 8 statements in in the formula? Or is there an easier and smaller way?.