In any case, make sure you number your invoices in sequential order so you don't confuse yourself. If you choose the latter, you can add '-01' to the end to differentiate between invoices sent on the same day. You can number your invoices numerically (1, 2, 3, 4) or by date (for example,, for the year, month and day of the invoice).
Since you are the one sending the invoice, you may choose the numbering system that works best for you. Write the following information on separate lines on the right side of the page: X Research source On the right side of the page, across from the recipient's contact information, include the details of your invoice. Write the invoice number and other invoice information.