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Your invoice ID consists of the letters "abcdef" and the digits 0-9 and may include dashes and was contained in the email you received when you last paid for Ardour.
It is not your PayPal "Transaction ID".
Examples:
0e2773e2-9b87-42e7-92bd-f7e399b09fbcor
91dc4e8fafdfbc8cf4a6ad0012240fe3