When sending multiple international parcels to different countries the system doesn't allow you to enter the correct zip code for your current parcel.
The zip code mask of the previous country is still applied when the newly selected country doesn't have a mandatory zip code mask.
Return to the dashboard and restart the process by choosing "New Shipment"
The behavior only happens when selecting a country without a mandatory zip code mask and the previous selected country had a mandatory zip code mask.
If the new selection also has a mandatory zip code mask this doesn't happen.