At fast food restaurants, two window clerks are given different instructions. The second clerk consistently sells more pie than the first clerk. Something vs. something beats something vs. nothing. You might argue that, over the course of the next 12 months, neither customer uses more stamps than they would have otherwise, and that the study was a total waste of time. But selling software isn't like that.
People can always use an additional software program.
There's Inventory Pro that handles both billing and inventory. They all sound great, and the web site visitor can choose among them. Having a high-end product to complement your basic products gives your customers an upgrade path. It also allows them to take their time in upgrading. They're comfortable knowing that more features are there if they want them.