The single most effective thing you can do to protect a domain.
ENABLING IT
Domains > My Domains > click the domain. Enable auto-renewal. Billing > Payment Methods. Add a card and set it as default.
Domain auto-renewal is a separate setting from hosting auto-renewal. Enabling one does not enable the other. Check both.
WHAT HAPPENS
The invoice is generated as usual and you are notified. The saved card is charged on or near the due date, and you receive a receipt. The domain renews without your involvement.
KEEPING IT WORKING
Update the card before it expires. A failed auto-renewal behaves exactly like no auto-renewal, and produces the same expiry sequence. Check the billing address matches your bank's records. Keep your contact email current so a failure notice reaches you.
WHY IT MATTERS MORE THAN FOR HOSTING
A missed hosting payment suspends a service you can restore by paying. A missed domain payment starts an expiry sequence that ends with someone else owning your name.
CHECK IT ANNUALLY
A five-minute review of auto-renewal settings and card expiry dates once a year prevents the worst thing that can happen to a website.