Update of this earlier post.
Heys called: "We are going to give you a replacement bag. We very rarely do this, because what happened to your backpack is not covered under the warranty, it is normal wear and tear. But since you bought it at ShopHeys-dot-com, and since you are a good Heys customer, we will replace your backpack. You may come pick it up anytime during normal business hours."
So when it finally became clear that I was not going to go away quietly, the company came to their senses and did what they should have done in the first place.
Obviously they had no intentions of honouring their warranty and were simply going to blame the problem on the customer.
Can this be considered good customer service? I'll leave that to your judgement.