For packages to be included in a USPS SCAN or End of Day form, they must have a ship date that's the same as the form's creation date. For instance, if you have a shipment with a ship date of 8/25, it can only go on a SCAN form that also has a date of 8/25. Additionally, the shipment must not have been on any previously created SCAN forms in ShipStation.
There are some measures that you can take to make sure that every shipment has the correct ship date. First, you can specify that every shipment created after a certain time be forwarded to the next day. The setting shown below is in Pacific Time.
Head to your Account Settings (the gear icon in the upper right).
Next, select Printing and then Printing Setup from the sidebar on the left.
Now click on Document Options for your Labels.
Additionally, when you actually ship the order, you can choose the ship date.
Finally, while you cannot change a ship date of a shipment after it has been created, you can void the incorrectly-dated shipment and then create a new shipment for that order. This is obviously a less-than-optimal solution for USPS, so we highly recommend that you configure the first two settings so that this third option becomes only a last resort. If you are creating a UPS or FedEx end of day form, voiding is less of a hassle since it is post-paid postage, so this last option may seem a bit more viable. However, we still recommend that you configure the first two settings according to your schedule.