There are several ways to do this. Either email, SMS, or print an invoice.

You can preview an invoice prior t sending it by email. Open an invoice > Share icon > Print as PDF.
When everything is okay, email the invoice. Open an invoice > Share icon > Email.
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There is an email template used to generate the email message. You can find this template in Invoice > Share icon > Modify templates > Email templates.
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Open an invoice > Share icon > Send as SMS
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There is an SMS template used to generate the SMS message. You can find this template in Invoice > Share icon > Modify templates > SMS templates. If you can’t see any SMS template yet, try sending one invoice as SMS (just send to yourself), then go back to the SMS template list.
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Open an invoice > Share icon > Print as HTML (useful if you don’t have an internet connection)
Or, open an invoice > Share icon > Print as PDF (needs internet)

You need a 3rd party printing app like PrinterShare, PrintHand, etc. You can search for these apps on Google Play. And of course a mobile printer.