Lydia never (ever) actually debits €1 when adding your credit card to the app.
Whenever you add a new card to the app, Lydia actually sends a request to the bank associated with that card in order to activate the 3D Secure process. This is a security measure to protect against theft or unauthorised use of your card.
3D Secured was developed several years ago to protect online card payments. Like many other companies, Lydia is obliged to carry out a test transaction.
This is the €1 transaction you see after adding a card. Once requested, this card authorisation is immediately cancelled.
Lydia never actually charges you this €1.
A very rarely visible operation
Most affected users never actually see this activity, however some cards, such as prepaid cards, are debited immediately, and notify users in real-time that Lydia has executed this authorisation.
Similarly, some card providers do not recognise the cancellation of this card authorisation requested by Lydia, and in fact wait up to ten working days before cancelling it.
During this time, €1 is frozen on the card, but remains in the user's account. This money is never sent to Lydia. This transaction will automatically be cancelled.