hmm.. this would require a bit new function. the online payments that get paid already send the notification email for payment, but that does not happen for offline payments of course.. a bit of coding would be required for this.

Dear Geminal,
'a bit of coding' will definitely mean requesting custom coding work right?
I will message you privately about it, but I must tell you, it is with great reluctance.
Because it seems Joomlathat did not consider that there will be other environments that do not use Paypal as their default payment method.
In fact, in Nigeria, 90% of the populace trust offline payments.
If this offline payment had the receipts inbuilt, any other request for payment gateways can be at an added cost through your custom coding service.
Messaging you in a bit.

Dear Germinal
I note that this subject is dated 2015. It is now mid 2017 and the receipt feature still does not seem to be present. I should have investigated this more deeply before adopting Invoice manager. I am now committed and our accounts team are having to receipt every transaction manually. As there will be up to 500 transactions a month that is a lot of manual graft.

As I said a lot of manual work - we are all unpaid volunteers at this time!! Some more custom coding I guess.
F