As a default settings, our Billdu app sends documents from our mailing server. Emails come from the email address firstname.lastname@example.org with your business name displayed in front.
Example of an invoice in Gmail
The email message comes with a blue View invoice button. When clicked, a window with your invoice opens. If your customer doesn't want to open the link, they can click on "Click here to download PDF"
Your customer can download a PDF file, pay the invoice, (if your card payments are turned on), create a delivery note, accept an estimate/quote or print it out.
The replies of your customers are routed to your email address.
Take a look here to learn more about sending documents from iOS.