If you truly want to express your appreciation, attach a handwritten note at the bottom of the invoice thanking them for their assistance. "Thank you for allowing me to be a part of your team for the previous five years," for example. The point is to let them know that you are grateful for the opportunity and that you appreciate everything they've done for you. Invoice writing is an important part of business, especially if you are new to it. Invoicing helps you close deals and get paid. It's also a way for you to show gratitude to those who have helped you out during your startup phase. So next time you need to write an invoice, look forward to it!