A card is required for both plans. Choosing a plan is the first step of registration.
Billing questions
When am I charged?
Pro creates a subscription with a 7-day introductory period; the first charge happens automatically when those 7 days end. Max has no trial and is charged on the day you subscribe.
Why do you need a card for both plans?
The card is stored to start the subscription so your alerts never pause between the trial and the paid period. You can remove the plan by cancelling before the trial ends.
Can I switch between Pro and Max?
Yes, from your portal at any time. Moving from a Pro trial to Max starts the charge immediately, because Max has no trial period.