Question: What’s a snappy comeback to someone who declines your invitation?
my honest answer: There is only one correct response to someone who declines an invitation:
‘I’m sorry you won’t be able to join me/us!’.
And then move the conversation along.
To require a snappy comeback implies that you are punishing them for not attending.
That means that you weren’t extending an invitation, you were making a demand.
That makes you an idiot.