Custom PriceLevel in new SalesOrder

Hi, I am writing an integration between NS and a billing solution. I can create new orders from the billing charge documents only if I choose an existing/standard PriceLevel, such as BasePrice (as identified by an internalId = "1"), as such:

RecordRef price = new RecordRef()
         {
             type = RecordType.priceLevel,
             typeSpecified = true,
             internalId = "1"
         };

The intent, however, is to set a custom price level for each order based upon the price specified in the billing platform. From what I've read, this should be possible by setting internalId = "-1", but when attempting to write the order to NS I get an error response:

[Code=INVALID_KEY_OR_REF] Invalid price reference key -1 for item 860.