Yes, you can put a pee pad inside a soft-sided carrier with your pet. This is especially useful for longer journeys when your pet may need to relieve themselves. Make sure the pee pad is secured properly and doesn't bunch up, which could cause discomfort for your pet.
Also, consider using pee pads specifically designed for pet carriers as they tend to be more absorbent and less likely to leak. Remember to check and replace the pad as needed during travel to maintain hygiene and comfort for your pet.
Carrier rules can vary by airline, so confirm your airline's current in-cabin pet policy before you fly.