Use guest checkout whenever possible.

If you do need to sign up, give fake info, or highly redacted info. If I decide later they actually need it, I can update it in the account settings. Nobody's ever cancelled my account because I said I was born Jan 1 over 120 years ago.

Do digital purchases via some middleman, like a PayPal account backed by a credit card, Apple Pay, etc. Lots of sites have PayPal checkout flows and it saves you having to share your CC directly.

Come up with site-specific usernames and store them in your password manager. That way you don't build a stable cross-site identity unless you're doing it intentionally for branding.