I guess that if you're using a Chrome Book, you have a Google account... That being the case, you can access Google drive and use it's spreadsheet function to create an invoice template, which you can use over and over again...
Open Drive, Click 'New' and then Sheets...
A few years ago now, I managed to work out how to set up formulas, to a level that suited my needs... Can provide these basic settings if required...
Not sure how to generate an new increasing number for the document, but typing one in isn't a back-breaking task...
I've just set up a complete book-keeping system for a friend (including a profit/loss return sheet) so a simple invoice should be child's play...
You can tot up total figures and even allow for VAT charges (or variants of) and discounts...
Robbie....