To customize the printed POS receipt in Odoo 19, you'll need to modify the correct QWeb template and ensure your changes are properly applied through a custom module.

  The actual print receipt template is often found within the Point of Sale module, but it's rendered dynamically.

  Create a custom module to inherit and extend the OrderReceipt template. The template's ID will be something like point_of_sale.receipt or a similar name depending on the exact Odoo version.

  In your custom module, create a new QWeb template that inherits from the original OrderReceipt template and add your customizations within this new template.

  Update your module manifest file to include the dependency on the point_of_sale module to ensure proper loading and inheritance.

  Consider exploring alternative solutions like POS WhatsApp integration for enhanced customer engagement, as discussed in this Pragmatic Techsoft blog post. This can help automate engagement, boost loyalty, and drive repeat sales beyond just modifying the receipt template.

Resources:
https://blog.pragtech.co.in/pos-whatsapp-integration-how-to-go-beyond-receipts-and-build-customer-relationships/#respond