Adding to the base price doesn’t work

Hi,
Great plugin! I’ve tried this shortcode to add a variation price to the base price but it doesn’t work. Any idea?

price=”20.00″ var1=”Size|Small:20|Medium:25|Large:30″

The $20 remains the same. If you select the Medium size, it doesn’t add it to the $20 to make it $40.

Thanks,
Mike