Trial extensions are not product keys. You can't activate trial extensions. They can only be used for unverified trials (now). And when TurboActivate 4.0 is released (soon) you'll be able to use trial extensions with verified trials as well.
You need to give me more information. But I'm guessing you're telling customers to use trial extensions to activate your app (or they're trying to do that on their own). Either way that's not how it works. You can only activate with product keys.
Does that make sense? If not, can you provide a full way to reproduce what you're seeing (or what the customer is seeing).