As a rule, receipts from the 2017 tax return onwards must only be presented to the tax office on request.
However, you should always keep them in a safe place and show them if the tax officials want to see them.
It is not possible to submit receipts electronically together with the tax return. The tax office only accepts them by mail.
For more information about what documents might be necessary for your tax return and what you can do, if you don’t have the receipts, please read this article (At the moment, it is only available in German.).