Don't get to happy, that's how they do it, they will charge your account when it ships and it will balance out.
Just wondering why you didn't order from SERO it's only $288 and it's the cheapest Sprint plans (like $30 mo 500 min night/weekends free with unlimited data/internet and other pluses).
You can always send it back and reoreder
Don't forget you can also get the $75 Kellogs rebate (do a search) besides the $100 one.