How can I calculate Square payment based on another field?

Our group produces many events each year. We want to allow people to pay for more than one event at a time.

I have set up the "Pay by Check" section to work properly, using values for each event and a hidden total field.

https://www.visitandrewsnc.com/andrews-chamber/andrews-chamber-event-vendors/

But I need two more things:

(1) I'd like the Square CC payment ("Pay by Credit Card") section to work the same as the "Pay by Check" section.  I do not see an option for extended calculations within square.

(2) I'd like to give a $15 discount to both sections, if all three events are chosen (I have done this on the check payment).

Can you help with this?

Thank you!

 I actually got this to work by passing the total into Square.  It gets rid of the individual products, but since the form passes that information, it's not a problem.