Autofill is a feature of Safari and is not Apple Pay. The same card can be in Safari Autofill and in Apple Wallet app (or appear in Wallet & Apple Pay in Settings) but are separate and treated separately in regard to payments.
While a default payment method can be set for Apple Wallet, no separate default method is available for Autofill. The default for Apple Wallet, becomes the default for Autofill as well.
You’ve currently have several options. Set the default for Apple Wallet as the card you want Safari Autofill to use or remove all Autofill cards except the one you want to use.
If you have old and expired cards in Autofill you can remove or edit the cards by following the path,
Mac > Safari > Settings (Preferences) > Autofill > disable or edit cards
Set a default card at the following path,
Mac > System Settings > Wallet & Apple Pay > Defaults > set preferred payment method
Its important to point out the Autofill does not offer the same security and privacy that using Apple Pay checkout does. Autofill does not hide personal and financial information from the merchant.