you could do this using a calculation in the eCart object on the calculations tab.
assuming that the values being passed for the size select list are "XL", "XXL" and "XXXL", and that the name of the size column is "Size"
create a new calculation named truePrice as:
(abs([Size] == 'XL' || [Size] == 'XXL' || [Size] == 'XL'))?[Price] + 3:[Price]
then change the TotalPrice calculation:
[Price] * [Quantity]
((abs([Size] == 'XL' || [Size] == 'XXL' || [Size] == 'XL'))?[Price] + 3:[Price]) * [Quantity]
and in the cart display, replace the binding for the Price column with the binding for the truePrice.