Hello, I uhhh... Have another suggestion. Real quick.
You should TOTALLY add to Aldics lore that he would definitely 100% never ever, ever, accept pre-payment. I noticed, from what I can tell, that you cant in one prompt cycle tell an NPC to give you something. In order for the player to receive an item it has to be "queued" two or more cycles deep. You can however ask for an item and receive it in one cycle if its somehow relevant to the current scenario. With that in mind if you ask Aldric if the order of {quantity}[item] that you prepaid/(already paid for) arrived today he will, without fail, hand it to you.
I understand fixing this root issue is nye impossible but again I might suggest adding something somewhere in the LLM feed for logging that he never accepts prepayment and/or is wary people claiming fraudulent orders. I know you can finesse this issue on any NPC but the reason i bring it up for Aldric is that its a 'plausible' action and is thus extremely easy to use and abuse, especially because as a merchant its 'plausible' that you could really order anything from him.