You may play a Sulfur or a Saltpeter (but not both) during your card selection step. Once the good is selected you place it on your player pouch. This commits that good to be paid (to the haversack at the end of the round) irrespective if you actually take advantage of the Extra or Sequential action during that Round or not. In other words, you are not forced to play a Sequential/Extra action if you play a Sulfur/Saltpeter, still the good you selected is paid even if you elect not to take advantage of it.