I've used multibit quite a bit for my own coins. I haven't found a way to export the raw hex, but that doesn't mean it's not there. Anyway, dude says "blockchain is showing it as invalid" which sorta makes it sound like he tried to put something into the pushtx page, I wonder what he put there.
Possible, I understood it as error multibit gives. If you are indeed correct and they already have the raw TX for some reason it would help if we could look at it. IIRC the interface by bc.i is not relaying everything either while I never had issues with bitpays interface.