HTML Formscentral forms don't support calculations so unfortunately what you are hoping to do is currently not possible. A number of people have asked for calculation and it is something we may do in a future release.
It does calculate when linked to Pay Pal, not when a PDF. What I would like to know is if I can for instance have the first registration/fee mandatory then perhaps ask if they would like to register another person yes/no. If yes it takes to the next set of fields to register another person and collect fee; if no it takes you to the proceed to checkout and Pay Pal. Is there a way to do that on a Pay Pal form?
I found a work around I only made the first registration required and made a registration fee choice required for each registration. I did have to add each purchase field for each registration, it wouldn’t add the fee for each person otherwise. I also had to make sure under the Address Fields that I had the billing name, address etc. checked so it pulled the information for the billing from the correct place. A bit cumbersome, however it appears to be working as intended.