That's a pretty reasonable price, certainly if it's closer to 70 EUR.
Given that you also get the bits for punching the letters in, you can easily sell cheaper plates without them.

4mm sounds like the perfect thickness, I wouldn't imagine that would bend, while it's still light and compact enough. 

If you're ever going to release a version 2.0 of this, you might want to figure out a way to stamp entire words on.
Maybe through some kind of placeholder for individual letter bits?

Now you have to stamp one letter at a time, right? Seems that way, looking at the picture on your website.