Teapplix ActionShip automatically uploads tracking numbers back to the marketplace and shopping carts hourly after you ship the orders. (Please note for eBay Accounts, to ensure that this happens regularly without error, make sure that you monitor your token expiration date. Tokens expire in about 18 months and if yours is about to expire, please use the "Renew" button to renew them.)
However if you manually mark an order shipped on the marketplace or shopping cart, ActionShip will no longer upload the tracking for that order. You have to manually copy the tracking numbers from ActionShip and add it to the order.
USPS Mail Tracking Number
USPS mail shipped via First Class Letter, Priority Envelope, and First Class Flat don't have tracking numbers since they are not supposed to contain merchandise, and have no realistic tracking. However, the USPS carrier would issue a reference id that can be used to go to their website and track certain delivery status for this type of mail. ActionShip will save and display the reference id for those shipped orders, but will not upload the reference id to marketplace, except Amazon since it is mandatory to have a number associated with a shipped order.
For more information, please check out this USPS Website: