How can I perform custom calculations on my form?

Also, if I have three different events that total to one total due and each
event has a service fee based on how many they select and one of the events
has a higher cost and thus a higher service fee, can that be included in
the calculation for coming up with a total due for PayPal?

each event has a service fee based on how many they select

Do you want to calculate the fee based on the number of times an option has been selected? I mean, based on the number of existing submissions with  the same option selected.

If this is what you're looking for, then it will not be possible to achieve; however, if you only want to perform some calculations and get a total, that would be indeed possible. 

You can set calculation values to your options, please check this guide that will help you doing so: https://www.jotform.com/help/301-How-to-Assign-Calculation-Value 

If each option has a fee, you may set up the fee in a hidden field, then include it in the calculation on your widget. Please check this guide that will help you performing the calculation on your form: https://www.jotform.com/help/259-How-to-Perform-Form-Calculation-Using-a-Widget 

I hope this helps.