Authorize.net API bug with negative line items

Hey folks - the Authorize.net API does not allow negative line items, so when there is a discount in an order, the API call fails. e.g.

Error: The 'AnetApi/xml/v1/schema/AnetApiSchema.xsd:unitPrice' element is invalid - The value '-506.25' is invalid according to its datatype 'Decimal' - The MinInclusive constraint failed.

I fixed this by adding a getItems() method to the gateway which simply returns false, but you may want to fix it another way.