When I purchased my carrier from Vera Bradley, I got a backpack to match it! It is great for when we travel and it is just the right size for a few clothes, treats, extra collars, leashes and also proof of vaccination papers. PLUS, I can easily sling it onto my back if I am carry the carrier!
A back pack seems to be much easlier for me than the bag that I used to use. I like having the extra arm free to use!