I also have this trunk bag and love it. (Last week I transported a large head of cabbage from a farm stand in the main compartment while riding with a friend.)

I'd check with a shoe or luggage repair service, too. Zipper replacements tend to be pricey, though...it may not be worth the repair cost. Could you use it in its limited capacity on another bike? I often frequent yard sales with mine , and sometimes have to leave the pannier unzipped due to space constraints. While it's not ideal, I find that most items stay put until I get them home.