Please open Simple Invoice from the “Apps” section of your Shopify store.
Navigate to Settings -> Manage Templates
Click “Other Settings” header from the left menu.
Navigate to “Custom Invoice Numbering” and you’ll see two fields called Invoice Prefix and Invoice Start Number. Prefix field is optional and you can type any number to the Invoice Start Number field. For example, if you type “1” to the Invoice Start Number” field, the first invoice you print or download will be numbered as “1” and the second one will be “2”. Please don’t forget to set a start date like 01.01.2020.
As a note: If you set a start date like 01.01.2020 and print one of your orders from 2019, that order won’t be numbered with a custom invoice number.
After you finish setup regarding custom invoice numbering, please print one of your orders from your store to check the output. You’ll see the custom invoice number in the Order # field of your invoice template. You can always change the Order # into something like Invoice #. All text fields are inline editable and you can change any text by clicking on it.