Subtotal not calculated correctly

[djmacllc]

Go to http://www.uslightingcorp.com/store/page8.html
Add Col Hemp Select 13.5 oz to the cart.
From the Shopping Cart:
Select Tax: Residential – In Michigan – 6.00%
Press the Recalculate button
Press the continue shopping button

Go to http://www.uslightingcorp.com/store/page7-ss6.html
Add T1* LUV TF Double Bro 13.5 oz to the cart
From the Shopping Cart:
Shipping, Taxable Amount and Tax calculated correctly.

Subtotal = 36.00
Shipping = 1.00
Taxable amount = 18.00
Tax = 1.08
Total = 38.08

Click on Mix & Match 25% discount 1 – 40 Items
Shipping and Discount calculated correctly.
Taxable Amount wrong. For tax purposes it is removing the discount equally across both products, instead of the discounted product only. This ends up calculating the Taxable Amount 2.25 lower than it should be.

Subtotal = 31.50
Shipping = 1.00
Taxable amount = 15.75
Tax = .95
Total = 33.45

Should be:

Subtotal = 31.50
Shipping = 1.00
Taxable amount = 18.00
Tax = 1.08
Total = 33.58