If you have an estimate that you wish to convert to an invoice, follow this guide.
1. Tap on "..." to switch to the overflow tab
2. Tap on "Estimates & Invoices"
3. Tap on an existing estimate that you wish to convert to an invoice
4. Tap on the overflow button in the top right
5. Tap on "Convert To Invoice"
6. A new invoice will be created with the same client, line items and tax rate as your estimate.