I have never had a company reimburse me free shipping plus a discount if an item didn’t arrive on time.
I have also never seen someone offer a full refund for a broken zipper 3 months later. Especially on cheaper boots. Zippers on boots are picky, and a lot of people try to force too narrow boots to fit by forcing the zipper up. Also, boots that are too tall break in and slump and that puts extra wear on zippers.
I am not sure why, after a full refund, the OP can’t just go get new zippers.