In the Builder mode, select the Product widget and proceed to its settings. On the left panel in the Product Option section, click Add option:
In the window that will open, set the values and modifiers. In total, you can create up to 3 options with an unlimited number of values.
For each option in the Extra fee field you can set a modifier for the original price. The modifier is added or subtracted and can be a fixed value or a percentage of the product value. To use a percentage of the value, simply type the percentage using the % sign and to subtract a fixed value, use the minus sign. If you don't want the original price to be changed, just keep the Extra fee field empty.