Registration with "cart items"

I am about to do a build for a non profit that needs a way for people to sign up for services that have a limited quantity.
The needs are overwhelming and the services(haircuts etc) are limited so they are trying to manage what they can do with folks.

My initial though was building a rapidcart pro store and set up quantities and just set the cost at free. It’s in a smaller poor town so a lot of the “clients” are not very tech savvy so the process needs to be simple.

Any tips or advice would be welcomed.

you may be able to get away with the free version of Ecwid.
very simple to integrate and to use