I work near Safeway, so I just buy grocery every few days and buy what i think will fit my panniers. If I have bulky items like toilet paper, I just use bungee cords and strap them to the top of the bike rack. Unless you buy stuff in bulk at Costco, I don't see what the use of buying special trailers. They are expensive too! Although, this looks really interesting...
it's call bikehod Don't know how well it works. I've yet to see it in Vancouver