How to make a form to collect multiple donations

Hi, I'm building a membership form for Oakland Tech's PTA, and I don't see a template that fits what I'm trying to do.

I would like to offer the option to buy more than one membership at specific amounts ($5 for staff and students; $10 for parents, etc.)

PLUS

an option to donate more (and this would be a minimum suggested amount (but people can choose any amount)

AND

we'd like the payment button to tally the amounts and charge for all of them at once.

We will also include: boxes to collect contact info and a dropdown for people to choose volunteer opportunities 

A lot of the templates have multiple products, but I couldn't find one that included donation options. I've tried a template and building a form from scratch, both linked below. But both seem to limit me to requesting payment for only one thing.

THANKS! 

https://www.jotform.com/build/72070525984157