You can pay online in (mobile) web stores or in merchant apps that offer the Bancontact app mobile payment service. Where available, this will be clearly indicated on the paypage of the web store or merchant app.
You can pay for your shopping with the Bancontact app at all physical locations that accept mobile payments with Bancontact. Look for a QR code that bears the Bancontact logo inside or next to it.
https://www.bancontact.com/en/faq/what-is-the-bancontact-app-v1