We guarantee all of our products against defective craftsmanship and materials.  Items returned due to failed construction or materials will be repaired/replaced free of charge.  Damages due to normal wear & tear and other extraneous circumstances will be repaired at a reasonable cost.
Zippers are the #1 thing we see issues with.  99% of the time they are issues related to normal wear and tear and/or abuse, but are repairable.  Zippers used in outdoor elements are kind of like your chain, they need regular lubrication, and cleaning to run smoothly, especially when out in the elements of dust and the hot, dry sun.  We sell a product called Zipper Ease that works really well, but there are a lot of other products out there...in a pinch you could even use lip balm.  So, before mailing a bag to us for repair, brush the zipper teeth to remove any possible debris, then try giving the zipper a little lubrication to see if that helps things.