Take it with you to the store. Shopping without checking your 'inventory' first will cost you money. Suppose our pro decides that it's time to buy more. One key element is how much time does he have before running out. If something is needed 'right now' price considerations become unimportant.
Speedy delivery is essential.
To be able to make their purchases when there's plenty of time available. Then they can price shop. Use their negotiating skills to get the best deal. You face the same situation in your grocery shopping. You'll stop at the quickest place on the way home from work.