[ic] Multiple taxes based on an Item-Attribute

[Admin Account]

Hi list,
Using Interchange 4.6.5
I thought I was almost done with my catalog but my customer  informed me
that any contact lens that they sell without a prescription must have an
additional 4% parish (county tax) in addition to the normal 4.25% LA state
tax.  Taxes apply to customers in LA only.  Which will be several thousand
of their current patients.  The same colored contact can be ordered with a
prescription or with out.

I have set up, with Ed's help, Item-Attributes to ask for 3 extra variables
(Power, Diameter, and BaseCurve)on the flypage.

How can I force the customer to enter these variables for ever purchase on
the flypage. Customers will be instructed to put PL for Power since this
determines the prescription.  This sands for plain in optometrist lingo.

Can I then do some type of [if item-modifier power = PL] then add 4% to the
4.25% state tax else do normal state tax.  I found good info in archives
about how to set up multiple taxes for different products but did not want
to have to double enter items.

I am perl and javascript deficient and a Interchange rookie to boot
Thanks for any help