For a former consulting business, I tracked my hours in a Tiller spreadsheet. At the end of the month, I summed the hours and manually created an invoice that I stored on my google drive.
I had a separate sheet to log invoices including the amount, invoice number and url to the google drive.
In my Transactions sheet, I added a column where I could link a client payment to an invoice number. Another column would show an icon with a link to the invoice when an invoice number is included.
The invoice tracking sheet also calculated which invoices were outstanding and when others were paid.
Unfortunately, none of this is currently shareable.