[Plugin: WP-Invoice – Web Invoice and Billing] Add own custom Invoice Number?

There is a way. Under Settings, in the Business Process tab, make sure to select “Automatically increment the invoice’s custom ID by one” and save all settings. Only after selecting this option will you be able to change the invoice numbers.

While editing your invoice, click “Toggle Advanced“. Then click the “Custom Invoice ID” link to the right of Invoice ID. This gives you a text field to enter your desired number.

After saving your invoice, it should be all set.

Hope this helps.