Can the Herald put itself on top of your draw deck?
No, Herald cannot put itself on top of the deck by overpaying.
From the designer, Donald X. Vaccarino on BGG.

Q: OK, so when the &quot;when buy&quot; effect kicks in, is the herald you just bought in the discard pile yet? Or would it have to be a &quot;when gain&quot; like Inn to do that? i.e., can you pay 5 to buy a herald and put it on top?
A: When-buy, which includes overpay, triggers before gaining, so you can't buy a Herald and use its overpay to put that very Herald on top of your deck.