Can a customer cancel an order they booked for later, or do they have to call me?

They can cancel from their own order page until the deadline you set, as long as the kitchen has not started. The deadline is the slot time minus the cancel hours on Settings → Ordering rules, and the default is two hours before. A customer cancel gives back exactly what a cancel from your board does: stock, a coupon use, a loyalty reward. They get a WhatsApp saying nothing is owed and your managers' group is told so nobody starts cooking. Set cancel hours to zero to close this door completely. The Orders & delivery tab in the docs covers the customer's cancel.