How to allow users to select more than one product?

On my Warrior weekend form, i added a paintball option for $25..

The registration of $85 is required.

If someone wants to add the paintball, then the total should be $85 +$25, but the final payment ends up being just the $25.

How can i add the two together if the paintball option is chosen?

Your payment integration is currently configured to allow users to select one product only, you need to change it to allow multiple selections so your users can select both products at the same time and sum the total: 

I hope this helps. 

Thank you!