[ic] Negative Sales Tax with Discounts

[Matt Kerner]

I'm having difficulty with a store we're building currently with the  
sales tax calculation. The site has products that are both taxable  
and nontaxable. The site also applies discounts to the entire order  
based on a customer's group affiliation. When a nontaxable product is  
purchased with a discount the discount/tax code still subtracts the  
discounted tax information. When no discount is applied the  
nontaxable information works properly.

Example:

Facts (simplified):
Customer A gets a 20% discount on all items in the store.
Tax rate for customer A is 10%
Product Y costs $100 and is not taxable

Situation:
Customer A buys product Y for $100
An entire_order discount of $20 is applied

Customer A sees a total of $78 (an $80 subtotal and a -2 tax)
The negative tax matches the tax rate multiplied by the discount  
amount (negative).

This same behavior happens when a customer purchases a mix of taxable  
and nontaxable products from the store in that the sales tax is  
decremented for the entire discount, not just the taxable products.

Any suggestions? Tips on where to look? Have other people encountered/ 
solved this problem?

Thanks,
Kerner