Bear in mind that the match fee is published in each league's rules, and has to be as part of SCoR. Charging more than that listed fee, even if it is offered to you unsolicited, could be viewed as an act of misconduct by the referee. Only needs you to upset that team during the game and they complain to league or county saying that you demanded more than the set fee to referee, neglecting to mention that they offered it to you. Or the opposition finding out and reporting you, suggesting that in effect you have been paid a "bung". Yes, you will probably be able to defend yourself by showing the text or email offering the higher fee, but you've still not followed the rules of the competition.
As tempting as it might seem, it isn't a route I would be going down.
Someone put this screenshot on Facebook ...
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