Upgrade pack through Steam

Hi I want to upgrade to the Cryptkeeper Supporter Pack for $10 using Steam funds. However the upgrade option is only available on the website for me and both the in game store as well as the Steam store page only show the full price.

I was wondering if I purchase another 100 coin pack for $10 using Steam, would I then be able to upgrade to the supporter pack from the website for $0? Or is there another way to do this?