If you buy the goods in Nevada then there will be Nevada sales tax of 6.85% to pay. If you buy the goods mail order then there is no sales tax and just the shipping. Lots of vendors have free shipping and you can pay a bit extra to get overnight shipping so they arrive while you are there.
Marv Golden was very helpful when I did this and the additional shipping with Marvs discount meant it was cheaper to do this than buy from a local pilot supplies shop.
Its best to contact them a week before you go to make sure they have them in stock and to set up the shipping for a particular day.
ZA