Is there an additional charge to use Verizon Message+ over wifi outside of the US?
There is no charge to your Verizon bill for wifi use. There may be charges from the wifi provider, but it is not on your Verizon bill.
It won't add to your Verizon bill, but there might be a wifi charge at the hotel, airport, ship, wherever you are.
To clarify. If I am on WiFi out of the country there are no global data use charges for activity on the app?
To help ensure that you are only using wifi, turn on the airplane mode and then turn wifi back on. The airplane mode turns off all the radios, but you can manually activate the wifi radio while in airplane mode.