How to Enter Commissions Paid on Sales and Recording GST Paid

I use a ticketing agency to sell my events. Each ticket sold includes the agency's booking fee and I am paid the difference.

What is the best way to enter both the ticket sales (GNR) and the booking fees (GST) all in one transaction, so that the GST paid is recorded correctly and the amount received is the difference between the two?

The way I am currently entering it results in the booking fee being a negative sale value with a negative GST collected instead of as a purchase value with a GST paid value.

I have the booking fees set up as a Cost of Sales account