If a message that ends in a C instead of an X is not followed up immediately whit a message that says simply 'X', it suggests that the the original mistake was not noticed by the other party when they did their ISTRB (Immediate Sent Text Read Back).
C is also for Cookie, that's good enough for me.
Woman: Hey hun;-) For Sure! I can't Wait xx
Man: Sorry about the C Babe! I missed that on my first ISTRB XX