The BigCommerce Checkout Type must be an Optimized one-page checkout.
To ensure the checkout type is correct:
Go to: BigCommerce Control Panel> Advanced Settings > Checkout.
Under Checkout Settings, the Checkout Type must be: Optimized one-page checkout. See picture below.
The Store Country must correspond to the Afterpay account region.
To ensure the store country is correct:
Go to: BigCommerce Control Panel > Store Setup > Store Profile.
Under Contact Information, the Store Country must be United States.
Next, you should Add a Payment Option.
When you’ve done that, make sure your customers are aware of the Afterpay by adding some Site Messaging.
Finally to attract lots of new customers, join the Afterpay Store Directory.