Unless otherwise told by your vet, you should not give your dog (or any animal) human medicine. Please ask your vet for allergy medications for your dog.
You shouldn't need to sedate a dog for traveling unless the dog has car anxiety/motion sickness. If that is the case and you don't want to get sedatives from a vet, you can try dosing 1 mg per pound of the children's benadryl. However, of course, this is an off-label use for the product so use at your own risk, and just like people, some dogs it does not affect at all.