If you allow registrants to pay by mail or pay at the event, they will be able to register online without submitting payment. They will show on your EVENT DASHBOARD with a balance due.
If you use Strip as your online payment gateway, you will be able to collect payment directly through your EVENT DASHBOARD.
Find your registrant and click on their name or the DETAILS button. If there is a balance due, you will see a "Pay Now by Credit Card" button at the bottom of the details panel.
You may also want to consider ALLOWING registrants to make payments themselves. In your EVENT BUILDER, click on REGISTRATION SETUP, Turn Registration ON/OFF. Under REGISTRATION METHODS, click ALLOW.