OK.. just sent Hostess an email thanking her again for inviting us to the party, and telling her that the boys love their animals, and haven't stopped talking about the party since yesterday.
I won't apologize because I do think it would be more awkward (also, knowing Hostess like I do, she would then feel horrible that I felt bad.. yeah, we're a family of apologizers).
I'm slapping my own hand, though. I did think it was awkward when the parent paid extra to get the bigger animal, but didn't even think about the addons at the time. DS3 had actually wanted one of the animals that cost $XX + Y too, but I had told him no, and pointed out the animals that he could pick from.