When you create a label in the PayPal Shipping Center, your PayPal account will be notified so the transaction will show as shipped and include the shipment details, like the tracking number. Once this happens, your PayPal account will notify your customer automatically via email with the shipment details. You can track your shipments from the Orders page of the PayPal Shipping Center.
USPS First-Class Mail shipments using the Large Envelopes/Flats package type are not trackable. Tracking numbers will not be sent to your customers for these shipments.
Shipment Notifications Not Sent for Manual Orders
Shipment notifications are not sent for labels created without a transaction in PayPal. This is because neither ShipStation nor PayPal will have the customer details required to notify them.