HP toner part numbers ending with either an A (for standard capacity), or an X (extra capacity) will both work in the same machine. High capacity toner is encouraged because it is a better value per page than standard capacity, since you get more impressions for less money over-all. To check this out, divide the price by the yield, then compare.
This does indeed seem like sound advice. However, as I was recently placing an order with Corporate Express for toner for our stake's HP M3027x, I gave the sales representative the copier's model number, and she placed an order for the HEWQ7551A. She didn't mention the toner cartridge's model number, yield, or price. However, just before she was about to end the call, I asked her for the price, and noticed that it was about 40% less than the last time I ordered toner, and so I was able to determine that it was the lower-capacity "A" cartridge, rather than the recommended high-capacity "X" cartridge that we ordered last time.
It would be good for someone in LDS Copier Administration to instruct the Corporate Express sales reps to default to ordering the "X" cartridges unless someone explicitly wants the "A" cartridge.