This is a great post! Not only to hear about the great win, but to hear about the sponsor being flexible. So I would call that a double win!
And yes, those taxes can bite. Evidently I didn't calculate my state tax correctly on a 2011 win and now I owe $400. Although part of me wants to protest, the other doesn't have the time or energy, and at least it's $400 and not $4000. So yes, you must consider the taxes and it's nice to see that the sponsor is too.
In fact, sponsors who contribute to your assumed tax burden are the best of all. In one case, one of my large wins was undervalued by the sponsor to about half of it's worth...which means my liability won't be much if anything...and I won't make the same mistake I made in 2011~! My state has so overcollected this last year against my income...even with my win I should be seeing a refund!!
Anyway, congrats on the win, and enjoy all the great prizes you do get to use!
Hilary 