Alternatively, you may find it in the email provided by the payment processors. Depending on the method of payment, this ID is called differently.
1. PayPal identifies it as "Invoice ID" in their invoice slip.
2. Coingate named it "Merchant Reference ID"
In any case, this unique 16 characters ID number is generated for each payment transaction. We usually require this ID to identify your order, confirm the payment status, issue invoice and more.
*Other payment processors may call it different, but it will remain 16 characters number that appears on the receipt.