Well... now that you tell me that you want to support 3XL, 4XL, and 5XL, I might do this differently to make it easier.
You could remove all of the XXLPrice columns and TruePrice calculation fields entirely and change TotalPrice back to [Price] * [Quantity].
Then go to your form and add a hidden form element for the size and change the values in the size dropdown list so that they pass the price like:
<hidden name="FBB_Cart_Store_1_Size_Add" id="FBB_Cart_Store_1_Size_Add" value="Small">
<select name="FBB_Cart_Store_1_Price_Add" onchange="document.getElementById('FBB_Cart_Store_1_Size_Add').value = this.options[this.selectedIndex].text">
<option value="35.25" selected>Small</option>
Then use the lightning bolt to set the value of the price field to come from the list.
This way the correct price is passed in directly instead of being calculated.