Ok, so Long Sleeve adds $5 to the price.
1) in the eCart object, add a new column named "style" (NOTE: after adding a new column to the cart, you will need to edit any add to cart buttons behaviors on your site to have the code regenerated to include the new column)
2) in the add to cart button behavior, set the style column to be updatable as a select list. in the select list that is added to the page, add the short sleeve and long sleeve options.
set the price column to the regular short sleeved price
3) in the eCart object, on the calculations tab, create a new calculation named "realPrice" as:
(abs([style] = "Long Sleeve"))?[Price] + 5:[Price]
this is a short hand if statement that basically says
If the style is Long Sleeve return the Price + 5 Else just return the Price
so if it is long sleeve, it returns 20, if not it returns 15.
4) on the calculations tab, edit the Total rice calculation:
((abs([style] = "Long Sleeve"))?[Price] + 5:[Price]) * [Quantity]
4) on the cart display page, where it shows the price, replace that with the truePrice binding.