I can’t purchase anything, it keeps saying update your payment details, and even after the updates my payments are declined

This document explains how to 'Change, add, [review,] or remove Apple ID payment methods' on an Apple mobile device, a Mac, or a PC; click here --> https://support.apple.com/HT201266

- If you are updating an old card and have ongoing subscriptions you may need to first add the new card and make it the primary payment method before deleting the old card.

'If you can't remove your last payment method or use no payment method with your Apple ID' click here --> https://support.apple.com/HT203905

If you still have problems Click here --> https://support.apple.com/choose-country-region/media-services Select your country, then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

Contact Apple for support and service --> "See a list of Apple phone numbers around the world." - https://support.apple.com/HT201232

Additional resources:

'Payment methods that you can use with your Apple ID' --> https://support.apple.com/HT202631

'If your payment method is declined in the App Store or iTunes Store' click here --> https://support.apple.com/HT203005

Apple Support Communities is set up as an end-user to end-user technical issue support service (i.e., users like you helping other users). Feedback for Apple will likely get more attention if you send it using:

Click on this link for Apple products feedback links --> http://www.apple.com/feedback/

Twice I've been into an Apple Store and purchased an iPad and covers and payment has gone successfully and now when I'm back home you are saying to update the payment details, which you can't and it's still declined. Do you not want custom because at this rate I will go to John Lewis as it's so much simpler to buy the item.