It's tough to say where the conversion value came from but the only options seem to be either a sale from the store or a sale that related to a default value (which you mentioned there is none.)
Without seeing it all and testing, it's very tough to determine. You should be able to tell from within Analytics what products made up that transaction.
I would suggest isolating the day the transaction occurred and then drilling down on the data to find out what made that up. That may provide some insight as to what caused it. You should even be able to get the order number etc... from within your analytics as well if that is setup. This will let you know what number it came from (if any) etc...
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