Update (2015-08-13): If you are using the 2013 R3 CU9 demo image you may run into the same error with a (much) different root cause and workaround. Please see this post for notes on how to fix that issue.
If you are using the Dynamics AX demo image (https://technet.microsoft.com/en-us/library/dn741578.aspx) you may have seen this error when adding an item to a transaction in EPOS, especially when logging into Windows as the Contoso\administrator user:
While there is a stack trace in Event Viewer that points to a problem the pricing engine, both the message and the event are somewhat misleading. Here’s what’s going on.
The problem is in the initial POS.exe.config file that is on the demo machine. If you remember, we separated out the pricing engine in EPOS so that it shares the CRT (commerce runtime) pricing engine with Call Center, MPOS, and the online store. The POS.exe.config file is missing the CRT section:
If you log into the system as Contoso\Emmah you can solve the problem very easily. You’ll notice that she has a bunch of shortcuts on her desktop:
Most of these are alternate ways to launch EPOS. If you were to dig in behind the scenes you would notice that each one is actually a simple Windows Batch file. Each one corresponds to a POS.exe.config file for a store/terminal. The batch file simply copies one of these configuration files over the existing POS.exe.config file and launches EPOS. Luckily each one of these is a valid configuration with the CRT section.
As long as you log in as Emmah and launch EPOS with one of these batch files you should be good to go moving forward. Emmah is also the user that has MPOS activated in the image so as a general rule you should log in with her account for all of your MPOS and EPOS demos.