The sales tax in Winfield couldn't be much higher than Tulsa; even at 10%, your tax for an 8lb jug of Unique would be $9.85 ($98.50/8lbs). Gas + tax still wouldn't add up to the $25 hazmat + shipping costs.
Kansas sales tax is lower (about 6-6.5% IIRC). I don't call 8 pounds (1 jug) of powder a group buy. It might be a nice start for one person in a group buy. Run the numbers for 30 pounds of powder and 20K of primers then tell me driving there makes sense. That amount of powder and primers should be easy to reach with several people stocking up. The key words are "group buy" as the original poster indicated. Vehicle gas mileage and time away from something else are other things to consider.
Just run the numbers and do what makes the best sense for a particular order.