Use your custom code from the AFTPAY offer to bring the price of $20 worth of stamps to $12.20, or 39% off (unfortunately the official Australia Post eBay store was ineligible for the offer).
If you are unable to find your custom code, view a product like this, scroll down to 'Pay only AU $85.00.', and click on 'Show me how', copy your code, and then enter it at checkout with the original product, and pay only $12.20 via Afterpay.
Why is it worth $22 rather than $20?