I am trying to product a wave template for WMS that will give me orders that have only one item. I have tried to use summary criteria to filter out orders with more than one item, but that produces an error when trying to search through the actual "Create a Wave" page. Any thoughts?

It looks like summary results are not supported (I get the same result). So unless NetSuite or anyone can recommend a better suggestion you'll need to implement a workaround. In my opinion the easiest solution would be to store the item count in a custom body field on the record using a script, and then filter on that custom field. The script could be user event or MR/scheduled. If you cannot script, then this can also be done with a workflow (event driven and/or scheduled) with some additional effort. CSV updates to that field would be required for existing records.

That seems like a reasonable answer...I will look into the script