Susan:

Given that you are not suppose to be able to add a misc charge to an existing receipt, either your issue description needs revision or you have found a way to add a misc charge (only in specific circumstances) that no one else has.

Are you actually adding a new misc charge to the receipt of just entering (adding) an amount to an existing misc charge?

I would agree with you that PO should have the ability to add a misc charge to any open receipt (delivery note) at any time (either as a new line or but entering the amount in an existing one).

Given that their are few users of PO and very few of them using the multiple vendors on a single PO, I doubt that anyone else will have come across this.