I know most Christians and Jews these days don't count as being part of Ahl Al Kittab, but that's irrelevant.
People can claim to be whatever they want, I could claim that I am a tyrannosaurus, but that doesn't make it true does it? The fact is that as per orthodox Sunni Islam, most of the Shia don't count as Muslim. Their concept of the 12 Imams, their insults towards the Sahabah (May Allah Be Pleased With Them), and their praying to graves puts them beyond the pale of Islam. You might say not all of the Shia are like this, but I guarantee the overwhelming majority of them do at least one of those things.
If it will offend them so much, just take the meat and throw it in the bin later on when they're not looking. Better yet, give it to a homeless person (assuming he's not a Muslim of course).
Someone taking the Shahadah is not enough to make them a Muslim, that's just the first step. Being a Muslim is submitting to Allah (The Most Gracious and The Most Merciful) by following his words and the words of his Prophets (Peace Be Upon Them), but declaring the Shahadah is just you acknowledging that Islam is the truth. They are two different things, and you need both of them to be a Muslim.
You claiming to be a tyranosaurus is not the same. the point here is, you cannot check whether someone is truely a Muslim or not.
What would you do if your halal butcher where you always buy your meat turns out to be a kafir? he just pretended to be a Muslim so he could sell a lot of "halal meat"?
You never know who is truely a Muslim and who is not right?
So, since we cannot look into peoples hearts...and we cannot know their true intentions...we must rely on his words. If he says he is a Muslim, then we have no right to doubt that...unless of course there are other reasons for doubt.
people who reject the Quran are kafirs per definition but appearantly still reliable consume their meat if they are Jews or Christians...We do not have to check whether they really are Christians, do we? but if we did, how would we know? do we then have to have knowledge about the Injil and Thorah in order to judge wwhether we can buy meat from a Christian or Jew? sound kind of ridiculous don't you think? Or do you think that is the intention?
So why is that different from Shia's? is a Shia worse than someone who openly rejects the Quran and our Prophet SAS?
So I think, there should be drawn a line somewhere where you can say...hey, this guy claims he is a Muslim / Christian / Jew, he is really nice to me...I have no reasons to doubt him, so I can buy and eat his meat without worrying.
Besides, you are talking about "most of "Shia's are not Muslim". most of them, means not all of them. and this alone already invalidates the given fatwa, that Shia meat is not Halal.
And then at last: do you think you are allowed to give haram food to a non-Muslim? Don't you think this non-muslim will complain about you in thehereafter that you have given him haraam food?