hello, everybody.
I want make commercial pc game product, but I don’t know how to make active product, example, player can download free, if he will play all of the levels, he need buy the game.
When customer purchases the game product, he will got an e-mail with S.N.
input the S.N. to game product, then the game can be active.
I hope to know how to do, Thanks friends.
yours
Shawn Zhang
you would commonly license a technology that offers you to do this and protects your game and hook it up with your game.
Developping something that works on your own is a pretty massive work and I doubt the effort is really worth it given that there are $400 solutions that just work well and which you then could use as base to implement them in your project with Unity Pros plugin capabilities.
You can license the system I made, which is less than $400. I used this system for Fractscape, which operates in demo mode until you buy a registration code using PayPal. Once you set it up, it’s completely automated. But yes, it’s a fair amount of work, especially since you can’t really afford to have any bugs at all. So you should license my system. 
It’s less expensive than others, and it’s the only one made specifically for Unity, although most of it is server-side.
–Eric