How to set the default card for iCloud subscription?

and my money was taken from two cards, even though I selected one card for payment.

Do you also get 2 x invoice from Apple?

If you have an iCloud subscription, you will receive a monthly bill from Apple, check your email box.

Sign in to your Apple ID account page and edit / update your Apple ID details -> https://appleid.apple.com                                                     

• On the Sidebar click Payment methods -> then tap Payments Methods and tap Manage payment method.

Or see -> Add a payment method to your Apple ID

Add a payment method to your Apple ID – Apple Support (UK)

• select one card for your default payment method.

Get more help and assistant from Apple directly, see Contact Apple for support and service -> Contact Apple for support and service – Apple Support (UK)