The product table has a field in it for Postage A,B,C, D etc and this references the corresponding field in the postage table to match off via a join.
So if a product has a postage rate of A then via the postage table this will produce a postage rate of $1
If anything else in postage rate A is ordered it will add no charge
If a product has a postage rate of B then via the postage table this will produce a postage rate of $1.50
If anything else in postage rate B is ordered it will add $0.50 per item after the initial $1.50
Postage rates are held in a database table and the column name for Postage A, B etc is PostageName.
The Postage Rate is held in a column name PostageRate.
The Additional Postage Rate for additional products is held in a column of PostageRateAdditional.
So i would be calling the Name (A,B etc) from PostageName, The rate for 1 product from PostageRate and the additional rate for any product over quantity of 1 from PostageRateAdditional