Stripe doesn't support '0' for price. The only workaround is to have 2 forms. Only users who would like to make a payment should be redirected to the "payment form". You can set this up workflow by redirecting users to an "Airtable Field". That field should contain a formula to filter the submissions based on their answers.
You can also use this workaround to only redirect to Stripe when a particular payment method is selected. For instance, if a customer select cash, they would skip the checkout form.