Experienced this today, and think I know why it is happening to me - It has only happened to me when I tried to buy something from a particular online clothing shop I have used before. Last time I purchased (12 months ago), I noticed that they actually set up an automatic payment pre-approval with Paypal rather than simply processing that one transaction. A pre-approval is used for things like subscriptions, where you pay something regularly (Spotify, Airbnb, Netflix all do it for example). When I saw the pre-approval in Paypal, I revoked it at the time (I didn't want an online clothes shop being approved to process my credit card in future, why would I? Surely I'm not that lazy!). Little did I know the trouble it would cause me later. Fast forward to today, and I try to make a purchase from the same merchant, and Paypal prompts me to link a new credit card in the checkout process. I believe that once I had logged in to Paypal in the checkout process, Pypal saw that I had previously revoked the old pre-approval for automatic payments linked to that one particular credit card, and so it wanted me to link a new card so the automatic payments pre-approval could be set up again. Now the sad news - it's not possible in Paypal to reactivate an old payment approval that has been revoked or expired. But the checkout process for this particular merchant does not handle that situation at all (since it's during the shopping cart checkout), and their account management page does not show any stored Paypal automatic payment approvals. The end result for me - I had to use the credit card directly and skip Paypal checkout. Ironically I used the same card I also have linked in Paypal that works just fine through Paypal with other merchants, who choose to be sensible and not trick you into doing an automatic payment pre-approval. This might affect some of you who are highly frustrated by this problem - log in to Paypal and see if the merchant causing you trouble is listed as an Inactive automatic payment (under Settings | Payments | Manage Automatic Payments). Good luck, hope this helps at least one person.
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