To use methods such as Afterpay and Zip, these payment methods would need to be shown on the item's listing, located underneath the item's price, as shown below. The payment method will only be available if all items in the trolley display the symbol of the Buy-Now, Pay Later method you're wanting to use.
These methods have a minimum and maximum order limit.
For Afterpay, your order needs to be between AUD $100.00 or over $2000.00. You can read more about this here.
For Zip, your order needs to be between AUD $20.00 - $2000.00. You can read more about this here.
Zip is also not eligible for pre-order items, so if an item is on pre-order, Zip will not be available at checkout.