It should be fine. My understanding is that the concern with purchased PFA or formalin is that methanol is frequently added as a stabilizer. In rare cases, this may quench endogenous florescence or otherwise mess with your tissue. However, according to the description on Affymetrix website there is no methanol in this preparation so this issue wouldn't apply. In a pinch I've diluted purchased formalin in PBS for fixation and it's been fine, although my typical approach is to prepare fresh 4% PFA in large batches which I freeze in aliquots to defrost as needed.