1) Before you can call your field under actions, you have to bring it "into scope" under conditions. Select "Entity has field". Under data selectors choose "commerce-product (saved commerce product).
2) For some reason, it seems impossible to use the "Multiple the Unit Amount by" option, as no matter what data selector you use, you will get an error. Instead select "Calculate a value"
3) Under "Input Value 1" select "commerce-product:commerce-price:amount".
4) Select which calculation to use. In my use case, I used multiply (*).