Hey Bryan,
Thanks for reaching out to us!
To set expectations here, we are limited in the support we’re able to provide for customizations such as this. A great place to request this type of feature would be on our UserVoice page.
However, to help get you pointed in the right direction here, you could accomplish this by following our Themer’s Guide to copy the “tickets.php” files for RSVP and WooCommerce (or ecommerce plugin you choose) and add them to your theme. From there, you could replace the quantity fields in those templates with a checkbox field.
Let me know if this helps.
Thanks!