Where are you going to be living? It's not as simple as just trying to get the cheapest retainer. Also, I believe invisalign are braces that go behind your teeth unlike traditional braces. Your husband will need follow up appointments to make sure that things are moving properly. So, even if the work is started one place he willl need further appointments to make sure things are moving correctly wherever he ultimately lives. Take it from someone who has had to have braces twice. Your husband should have the work done where he lives and his dentist should be able to refer him to an orthodontist if that is who he needs to see.