sales invoices with ncg code

Hi, I'm using Reckon Accounts Hosted. 
I have to enter a Sales Invoice that includes: 
- "Receipts Income" with FRE code
- Negative "Service Fee expense" with NCG code
However, when I run the GST detail report, the Service Fee appears as a negative Output (Sales) as opposed to an Input (Purchases). 
Is there a way to fix this? 
Many Thanks, Brianna