Why Was I Charged for a Declined Transaction?

It depends on whether the transaction was truly declined or still pending:
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If a transaction is declined, you aren't charged. No funds leave your account. You can check this anytime in your transaction history on the app's home screen.

Sometimes an amount is held for a transaction that doesn't go through. If that happens, the held amount is usually refunded within 2 to 5 days, so there's nothing you need to do.