How do I create an invoice without product? Am working a project that required generating invoice for customer service payment, 'rent' to be specific. Am still a newbie to drupal environment (I do Wordpress Stuff). 

Since the Payment is going to be recurrent I choose Drupal Commerce, but am confuse as to how to configure the module(s) needed.

Features that I need
- Admin created invoice for Client (registered user) with invoice status UNPAID
- Invoice sent to Client via email (PDF format)
- Client Pay at Bank and upload Prove of payment
- Admin change the invoice status to PAID upon Validation.

I could Create Content type for the invoice, but I may not be able to create PDF and mail it to user in which Drupal Commerce would handled well with other Contributed Module. 

I should be able to handle it if I can get a kind of guide.
I would take a look at the Commerce Billy module

Features

Thread safe invoice number generation
Three invoice number generation methods: monthly, yearly, infinite
Configurable pre- an suffix values for invoice numbers
Arbitrary start value for invoice number
"invoiced" status on order entity (thus avoiding separate invoice entity)
Automatic or manual invoicing:
    Automatic: order moved to status "invoiced" on order completion
    Manual: admin moves order to status "invoiced" (VBO possible, Rules components already included)
PDF Generation (commerce_billy_pdf module)
Customizable PDF template
Views integration for PDF download link
Handling of credit memos (canceled orders)