The website lists a maximum
rider weight of 220#.
http://community.bikefriday.com/faq?...1703&x=18&y=14
Could be that the seat mast is the real limiting element such that the pannier weight matters little.
If you have a relatively large two-wheel trailer, why not move some of the cargo there? Or does the pooch -- a rather large pooch for that matter -- take up all of the space?