Price Option Calculation with Company Price Ref

I have tried various way to try and get this to work, but with no luck, so any help would be greatly appreciated

What I am trying to do is automate pricing options in the table 'Products' in connection with each Company using a price code when filling in the Invoice/Purchase Orders. So every time a company is set up in the table 'Customer Detail' the operator has the choice of giving each company a code which refers to different price options, or left blank for RRP. When opening a new Purchase Order / Invoice you have to fill in the Company Details from a draw down list which includes the 'Price Code' - A,B,C,D or blank. From there on any items added via the portal should reflect the price code option set up in 'Products' and 'Line Items'. I have created a calculation in the "single Unit Price ' field in the table 'Line Items" which works fine in isolation but via portals and look ups doesn't work most of the time. Oddly enough it works sporadically.

I have enclosed the file I am working with for clarification.

I hope someone can help