Behavior:

When sending multiple international parcels to different countries the system doesn't allow you to enter the correct zip code for your current parcel.

Cause:

The zip code mask of the previous country is still applied when the newly selected country doesn't have a mandatory zip code mask. 

Work around:

Return to the dashboard and restart the process by choosing "New Shipment"

Note:

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.