I need help creating an online sign up sheet with time slots that allows them to prepay online

I'm not exactly sure what you meant by pre-paying online. If your goal is to sell something that revolves around timeslots (e.g. appointments, scheduling, reservation, etc.), you need to be more specific on how this is going to work.

For now, the widgets I'm listing below track user selection for each submission. This would mean that when someone picks a slot/date/item/etc it will be counted towards the submission and the widget counter will reduce by 1. Feel free to check them out and pick one that will suit your needs.

1. Appointment Slots Widget

2. Date Reservation Widget

3. Inventory Widget

4. Ticket Purchases Widget

Related guide: How-to-Add-a-Widget-to-your-Form

In case we misunderstood your question, just get back to us with a bit more info and a clearer description of what you're trying to do so we can better assist you.