Normally on booking you are given an email address to allow you to validate your visa on line, but I don't know what the deadline is for doing this. I do know that if you don't do that, you have to purchase one at the airport on arrival at Cape Verde , it used to cost €15 but may have gone up since we last went. I would expect that if the online validation is not possible due to short notice, this should be reflected in the price of the holiday.