black sling bag (zipper problem) (Central Square, Cambridge)

I have a Red Rock sling bag, as pictured. This exact same model. One of the rows of zipper teeth has come loose from the other part of the zipper. I have taken this two repair places who have quoted at least $60 to fix this, which would require the use of specialized sewing equipment. (Glueing it didn't work.)

Please don't ask if I still have it "just to make sure". Once I give this away, I'll take down the post.