Most providers do not accept cash or checks.
One of the reasons providers offer online registrations is to go "paperless" and eliminate the administrative headaches of tracking payments, refunds, bounced checks.
You should contact the provider to see if they accept cash or checks. If they do, they will provide you with instructions on how to submit payment (e.g. where to mail the cash or check) and send you a discount code that you can then use to complete your registration.
Please use the "Contact" link on the registration flyer to request to pay fees using cash or check.
Programs that accept payment outs of the registration system can use the "Receive Non-CC" payment on the Registration Editor and provide the check number in the comments to record that the payment was made.