If you need an invoice you should always ask the seller.
You can see all your expenses on your feed and, by clicking on a payment you did (Services and State), you can see more details and have access to a pdf with the proof of the transaction, if it was made through the app. This is not an official receipt.
To have an invoice of your mobile top up (Telcos), select Invoice on the screen where you enter your top up details.
When you complete the top up you will see a screen with the invoice in pdf. You can also access that invoice through My moey!, by clicking on the transation for which you said you wanted an invoice.