Ok thanks. Actually wait, if a $5 donation gets you $10 off the final game, the FAQ says the final game would be around $15 so $10 off that would be only $5 anyway so a $5 donation would cover the cost of it. If it's a $5 donation and then another $5 to get the game then based on the cost mentioned in the FAQ it would basically only be a $5 discount because you would be paying $10 for the game in total, which would make it the same price as the Level 2 donator reward.
Donate $5 now and get the game for another $5 when it goes alpha and everyone must pay. Total final payment $10.
Donate $10 now and get the game for $0 when it goes alpha and everyone must pay... basically you prebought. Total final payment $10.
Donate >$10 and get other benefits as spelled out on the benefits page.
The level 1 option is for people who don't believe enough in the game to prebuy it even at the $10 level but otherwise would like to have an actual game account and get access to the donators section of the forum. Rather than penalize them later, I give them a discount equivalent of the $10 donator gets... they just have to pay the difference. The game account will let them (eventually) have a central login that should work across servers and will provide access to the (eventual) player mod store.
Another point of clarification, there may be some reason I end up charging more than $15 for the real game. The $10 donator gets locked in at the current price and definitely already has a license when I go alpha. The $5 donator only gets a $10 discount on whatever price the single player game goes for.