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Stand Alone Transaction cannot be used with this feature. Make sure Stand Alone Transaction is unchecked inside of the campaign product.
First, contact PayPal and have the Reference Transaction Enabled on their end if you are planning to enable rebilling or upsells with PayPal.
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Make sure you check the referenceTransactionsEnabledreferenceTransactions box on your PayPal MID setup to enable rebilling and upsells for seamless checkout.
After enabling the referenceTransactionsEnabledreferenceTransactions option on your PayPal MID, log into your PayPal merchant account and confirm that your account is configured correctly to support Reference Transactions.
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You can also send the fulfillment tracking numbers back to PayPal by enabling the sendFulfillmentTrackingNumber option and filling in the clientId and secret fields.
Add this code inside the form element on the upsell page:
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