2. How can I display a QR Code on an eCommerce Website or on an invoice in the customer's portal?

By enabling "SEPA QR Code" in Configuration -> Settings -> Customer Payments.

By adding a "SEPA QR Code" block to the Web Page Edition view.

>By enabling "Use SEPA QR Code" in a Payment Acquirer's form view, under the Configuration tab.

By activating the "SEPA QR Code" Payment Acquirer.

It should be 

By enabling "SEPA QR Code" in  Configuration -> Settings -> Invoicing -> Customer Payments.