going for it, I'm afraid 4KAL. This was a poor grid.

Another frustrating unimaginative grid where the exact same word is in a clue and a solution. "Beak is a beak". "An Orange is Orange" "DNA is related to DNA" etc.
In your grouping of forms of payment, I can only agree with one - VOUCHER. You could argue the various definitions of BILL - one could be a banknote. (things put in a billfold)
An invoice is usually a documentary request for payment, and issued to cover both parties accounting requirements.
A RECEIpt is usually generated by someone who has RECEIved something. You can't use a bakery receipt to pay for anything of value.

I suppose related to payment might have been better?

Entering "payment" does not get you a bonus - plus I agree the description is poorly thought out
Beak one is very hard, but agree that beak should not be part of it.

'mammal anagrams' should be accepted. I should not have to say 'small' and 'British'.