You pay tax at registration. If you are taking the vehicle out of state you aren’t required to pay state sales tax. You pay upon registration in your state.
There are a couple of stores that I would look at buying from Wysup CJDR in Pullman WA. They may sell you a truck at invoice, Kendall Ford in Bozeman, MT or their store in Meridian, ID. If you are looking for a GM Cascade Chevrolet or Sangster GMC both in Wenatchee, WA