Hi

Using your template

For me, the simplest way would be the following:

Using the example above if the customer still enters
A=4
B=anything
C=4/15/13
D=$1.00
E=$4.00
F="S" or "I" and it will automatically know 5% or 2% based on the letter
entered (make this a Data Validation so that they can only choose S
or I).

G=If(F="S",.05,.02)

H=(E*G)
I=(A*H)

HTH
Mick