Discount for selecting more than one checkbox?

I just purchased the Pro version because of it’s ability to have discounts. However now that I am trying to set it up, I don’t see what I need. I am having users register for summer camps on my website, and if they sign up for 2 or more, they get 10% off. How can I set this up? Each camp is listed – along with it’s full price – as a check box item.
Thank you!

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