An interesting way of thinking.
Surcharge meaning an extra charge over the basic charge is an established meaning with a long history. Originating from France, the word was adopted into the English language. I am not quite sure what makes the heraldic 'charge'
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"charge" in the older sense "mark, symbol"
Heraldic terminology is mostly French and not an adoption of French. It does not have widespread use.
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A charge is defined as, "anything borne on a coat of arms, whether upon the field ... or upon an ordinary, or indeed upon another charge."
Gough, Henry, and James Parker. A Glossary of Terms Used in Heraldry. (Oxford and London: James Parker & Company, 1894; republished by Gale Research Company, 1966).
Linking the use of surcharge to heraldry requires knowledge of heraldic terms, but ignorance when it comes to the use of that terminology.
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This usage is in no way incorrect
'charge' in he meaning assigned to 'surcharge.' Adding 'sur' is superfluous in heraldry. A charge on a charge remains a charge and does not become a 'surcharge.' Also, I am not quite sure how 'charge' translates into 'mark' in the sense of the overprint. This meaning of 'mark' puts the emphasis on it being a symbol, rather than an overprinted value.
If the heraldic use if charge is indeed the origin, below stamps posted by danstamps54 are surcharged.

This being the one example I have never seen referred to as 'surcharged.'