did you know a borax solution can be used to calibrate near this range? It will produce a buffer solution of around pH 9.2, so this method requires a controller that will allow calibration using arbitrary buffers.That Sparkfun kit doesn't do that.
Because it already has set calibration points. 4.0, 7.0, and 10.0 are very commonly used and widely available buffer solutions. And at $25, the stamp is the cheapest I have found.