You can customize the invoice layout by creating template overrides from the site frontend template and the administrator template. Follow the instructions below based on your requirement.

For Users (Site Template)

1. From your Joomla dashboard, go to System > Site Templates.

2. Select the template currently in use, for example: Nectar Details and Files, and click on it.

3. Select the Create Overrides tab and navigate to the Components section.

4. From the list, locate com_easystore, expand it, and click on order. The override will be created automatically. 

5. Navigate to your template’s source file, for example:
/templates/nectar/html/layouts/com_easystore/order select your invoice file to modify the invoice layout as required.

6. Next, from the same “Create Overrides” tab navigate to the Layouts section.

7. From the list, locate com_easystore, expand it, and click on invoice. The invoice layout override will be created automatically.

8. Navigate to your template’s source file, for example:/templates/nectar/html/layouts/com_easystore/invoice and modify the overridden invoice layout as required.

For Admin (Administrator Template)

1. From your Joomla dashboard, go to System > Administrator Templates.

2. Select the administrator template currently in use. For example, choose Atum if it is your default template and click on it.

3. Select the Create Overrides tab and navigate to the Components section.

4. Locate com_easystore in the list and select Order to generate the override.

5. Navigate to your template’s source file, for example: /administrator/templates/atum/html/com_easystore/order/ and select your invoice file to start modifying the invoice layout as needed.

Note: You can customize the invoice text directly from your template’s language file. 
For example, update the relevant strings in /administrator/language/en-GB/com_easystore.ini to change the wording displayed on the invoice.