fed ex sending me a customs bill weeks after delivering package?

Long story short, a product I used broke, contacted the company and they said it was a warranty issue and they would send replacements which they did. came home a few weeks back to it on my doorstep. now I'm getting a bill from FedEx claiming they want 60 bucks for it. I never signed or agreed to any such thing and nobody from FedEx contacted me to alert me or have me agree to it, they just dropped off my package.

in order to not pay this (which I do not intend to) what is the process? do I ignore it since I never signed anything or accepted anything? or do I have to call them and tell them as such?

furthermore is this even legal? normally I would assume its a scam but this seems to accurate on details to be a scam, prior customs clearance always required me to either agree ahead of time and pay/self clear, or pay to receive my goods prior to it being delivered. this was just left on my doorstep.

I'm just worried that ignoring this as a scam might end up as a bigger hassle later on so I want to tell them to buzz off but in the proper way. That is if its not a very sophisticated scam