Sounds reasonable - sort of. They don't have a real "guarantee" program - they're just covering their cost, figuring if they have to replace your battery, they've still made money on your juice purchases.
It doesn't change the fact that you can find better value online. Unless their juice is your FAVORITE, or they're providing some other valuable service (teaching you stuff about your next step vaping), then you should probably spend some time reading here, shopping around, and try some stuff from vendors you see recommended here!