I keep getting a notification of a declined payment

Your are not asking Apple here but rather other end users. We cannot access Apple's records to check anything on your behalf. There is a way you can check it yourself. Start here:

If you see 'apple.com/bill' on your billing statement - Apple Support

The most important part of that article is "View your purchase history." If Apple does NOT show a record of the charges in your history, it is very possible that scammers have gained access to your credit card or bank number and are pretending to be Apple. It happened to us.

If that is the case, only your CARD ISSUER or BANK can remedy it.

If your payment method is declined in the App Store or iTunes Store, see -> 

If your payment method is declined in the App Store or iTunes Store - Apple Support

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How to complete your purchase if a payment failed or was declined

1. On your iPhone or iPad, open the Settings app.
2. Tap your name.
3. Tap Payment & Shipping.
4. Add a different payment method and remove the old one. 
5. Try to complete your purchase.

If you still can't make a purchase

• You might have an unpaid order. Learn how to pay an unpaid order.
• Contact your financial institution. 
  • You might need to ask your financial institution to allow international purchases on your debit or credit card.
  • If you want to know why a debit card, credit card, or other payment method was declined, speak with your financial institution. Apple doesn't have information about why the payment method was declined.
• In some countries and regions, your payment method might be declined because of changes to local law. Learn about billing and purchases in Europe or what to do if your card is declined in India.
• Still need help? Contact Apple Support.

See the following Apple articles:

Payment methods you can use with your Apple ID

Payment methods you can use with your Apple ID – Apple Support (UK)

fyi: Not all credit & debit cards seem to work with App Store / iTunes Store, try a different one, or preferably a credit card. If your card is not accepted as a paymenf method, you need to contact your bank.

What you can buy with your Apple Gift Card or Apple Account balance

What you can buy with your Apple Gift Card or Apple Account balance – Apple Support (UK)

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• You can't use your Apple Account balance to send gifts or gift cards.
• If you use Family Sharing, your family members can't spend your Apple Account balance. Funds that you add to your Apple Account balance are only available to you. Find out more about how purchases are billed to your Apple Account balance.
• Some purchases might require you to have a credit card on file, even if you use your Apple Account balance to make the purchase.

If you need to change or update your Apple ID payment method

If you need to change or update your Apple ID payment method - Apple Support

• If you want to change your card, add first the new card, then remove the old one.
• or if you want to use your Apple ID balance, you need to set your Apple ID balance as your default payment.

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

• If you want to change your card, add first the new card, then remove the old one.
• or if you want to use your Apple ID balance, you need to set your Apple ID balance as your default payment.

Add money to your Apple Account balance -> Add money to your Apple Account balance - Apple Support

If you have a valid payment method on file, instead of buying or order an App Store & iTunes gift card, you can Add money to your Apple Account balance anytime, no gift card needed, see -> Apple Gift Card - Apple (UK)

If you have followed all the steps and instructions and if you still need help, get support from Apple directly -> Get Support or call Apple in your region see -> Contact Apple for support and service or check -> here for more support and assistance.