Add Section , Add Note in the Invoice fields

Hi chaps.
I love erpnext comparing to odoo. Its just one tiny thing missing from this ERP.
When I’m creating a quotation, or invoice there is just the option Add Item or Add multiple items option in the items list. I would love to have the option to add a custom note or even a custom section just like in Odoo. As we do large invoices we need to break them into sections/chapters and each chapter has a subtotal cost of the items within that section. That makes it easier for a client to understand what he is paying for.
For example : Section 1 (Product A, Product B, Product C , Service 1, Service 2). then a subtotal of that section and further section 2, section 3 with their respective items.

I understand you can add custom fields, but my question is how to create a button " add section" and when you press it to add a section under the last item line? and then to be able to move it up and down and the items under that section line will be calculated in the subtotal of that section.

Im willing to pay if there is a developer able to do that. Thanks.