When working with suppliers you need to consider the units of measure that you have setup in AX vs what the UoM the vendor is using. A different unit could mean a different quantity of a product. When the users returns with the shopping basket from the suppliers site the unit of measure is specified in the cXML message that returns. If the UoM is not setup in AX in the units table then when the user goes to convert the shopping basket to a requisition they will get an error.
When using the cXML punchOut protocol handler for AX2012 you will need to consider in your implementation the setting for the default unit. You will find this on the procurement and sourcing parameters, procurement site settings. This was provided so that the you can choose to show the user an error and but still allow them to continue to convert to a requisition with the default UoM. Once they are in the requisition then it can be manually changed.
This may or may not work for your procedures so you should consider how you want to handle this. One other way is not to allow the default units and trainer the users that when the see the error message in the shopping basked they should notify the administrator to setup the units in AX.
Here is a quick overview.
For notes on downloading the cXML punchOut protocol for AX2012 see this post