You can use a Custom Invoice ID for your invoices. To assign a custom ID to your agency account, go to Settings > Invoices, Offer and Payments > Invoices. You'll have the option to start with a new number or continue a existing one.
Once you create an invoice, you can click Edit and add a custom invoice ID.
If you have Payment Plans enabled, you can add the ID in the invoice preview.
Custom IDs will reflect on the invoice tab, contracts and invoices.