When you make a payment in the app, a payment receipt is displayed with Receipt number and payment date, which you can send to your inbox via the "share" icon.
We are not planning on introducing tax invoices in the app at this point in time but will continue monitoring feedback.
This is a feature I would like as well. Particularly given that the first time you pay a bill it is not obvious that if you don't email it to yourself you will not be able to get a tax invoice subsequently. I need a tax invoice and cannot get one.