What was a medieval cobblers job?
To make shoes
A cobbler's job included making shoes, but was possibly more often shoe repair, and was more generally working with heavy leather for all sorts of purposes. Cobblers often did not have permanent shops and travelled all over the countryside doing whatever work they could find.
The manufacture of fine shoes and work with soft leather was done by a person called a cordwainer. By contrast with cobblers, cordwainers usually and shops in towns or cities and had wealthy clients.
I'm not sure if cobblers fix zippers on boots,but I do know a place where you can get them fixed. It is in Mesa,Arizona and it's called Boot and Shoe Repair and it is located on 4815 East Main Street. I have tooken my boots here before for zipper repair and they did an excellent job, I was very pleased.