flying with old dog, paperwork

Butter is 10 years old now so I was worried about the flight. Our vet gave me a health certificate, she checked her heart and her eyes first. I need to bring the vaccine record too. The airline asked for her weight and crate size, and they check the crate at the counter. Cabin or cargo? For a small dog we do cabin, carrier under the seat. I bring pads and water because she pees a lot when nervous. Please check the airline website before you go, the rules changed for us two times this year, I was surprised.