ALL a/c that fly internationally must land at an international airport. From my understanding, by definition, an "international" airport directly means that it has customs irregardless of whether it has Part 121 (scheduled commercial), Part 135 (on demand / air taxi) or Part 91 (general aviation) traffic. So probably most international airports do not have scheduled commercial part 121 traffic. I don't know what the laws are and don't want to find out but if you land at a non-international airport after crossing a border, apparently it 'ain't good' as you are illegally entering a country and the interesting thing is that this is true even if the pilot declares an emergency. I don't know this from first hand info as I've never flown internationally...although I've passenger'ed dozens of times.