To enable Afterpay as a payment method and display the Afterpay assets on the product pages, do the following:
Log in to the Ecwid Website Admin Portal.

Go to Configuration -> Payment.

Contact Ecwid support here to help with the Afterpay setup and installation. An alternative is to open the Ecwid Control panel and use Open Live Chat or email support.
Enter the Merchant ID and Secret Key supplied by Afterpay. Ensure Test transactions (no real charges) is set to Disabled.

Double check that Test transactions (no real charges) is set to Disabled. See picture below:

Update the Appearance Options text, insert the text: Afterpay - Shop Now. Enjoy Now. Pay Later.

Set Enable this payment method at checkout to Enabled.

Click the Save button.

Afterpay should now be installed and available for use by your customers. Afterpay should also appear on your product pages and as a payment option on the checkout page.