Jason,
I have been testing my add to cart button today with the checkbox selection for golfer or foursome with the additional charge. The charges for both the individual golfer and the foursome are being added in the cart whether or not the checkboxes have been selected.
Here are my checkboxes...
<input name="LTACconvention_1_golfer_Add" type="checkbox" value="1" >
<input type="checkbox" name="LTACconvention_1_foursome_Add" value="1" >
and the calculation you created in my cart object...
True Price Calculation - (abs([golfer] = 1)?135:0) + (abs([foursome] = 1)?500:0) + [Price]
Total Price Calcualtion - ((abs([golfer] = 1)?135:0) + (abs([foursome] = 1)?500:0) + [Price]) * [Quantity]