I would interested in knowing the answer to this one as well as I’m U.K. based.
my thoughts on the matter are as follows, your only paying for the grading of the comic itself so any duty that you might incur would be related to that and not to the total value of the comic itself as you own it, if I was to send anything off I would keep all the records of the shipping of the item, mark it on the decoration form on its way to the states take a picture of it and so on, as we all know what customs are like they hit for the full value of the comic, but in your defence you could counter that clam with the evidence you have (ie your shipping documents when you originally sent it off ) and you are willing to pay customs duty on the service you have received bit not for the value of the comic itself as it belongs to you.
I’m worrying about the same thing I bought an item from Germany last November pre Brexit due to arrive March 2021 now would they hit me with duty on it as we now left the EU again my take on it and this will be my argument with customs should it happen will be I paid for this item when we part of the EU and should be exempt from any duty.
Please Note : This is the way I see it please do not take my advice on your above question as I do not work for customs and exercise and never had. Your best port of call is to speak to them direct and ask them what the go is.
