We have snuck our papillons on in the past but not lately. If you are caught, they can deny boarding (no reservation, too many animals already, not enough time to process collecting fee at gate, etc.). My guess is if boarding is denied Delta could treat it like you no showed and cancel the rest of the ticket.

Absolutely agree with the soft sided carrier, the space under seats is rarely a rectangular box so some give makes it fit easier.

The fee really annoys us for two reasons - the dog counts as one of your carry-ons and the airline does almost nothing for the money, just keeps track of animal inventory on the plane.