I dunno if you've seen other posts yet where this was addressed, but
My Recommendation on Customers asking for shields is to only provide them with a shield, or only with a weapon, and not both.
The game can be saved after they pay and you can reload to clear stuck npcs away - but - If you only give them one or the other, and not both, you can simply dismiss the customer with the X button that you see when looking at them, and they will pay for whatever you gave them.
I assume, while it isn't verified as I don't really know code and haven't really seen it.
That its essentially an issue with how new the change is - More than likely, The Customer is told to "Wait for item" - normally - This would be "satisfied with weapon/item" and they would leave - however a customer asking for item and shield is told "Wait for items" and when they take one they are reinstructed to continue waiting for the other item....
But then they get the other item, and they're reinstructed again, to wait for the other item (which they already have)
While at the same time, they've accomplished "Wait for items" - and thus, begin to "clear out" - Thus their order and X button disappear like they're about to leave, they pay out, but then realize they've been order to "Wait for the other item"
By this time "The other item" is "Null" "N/A" because they have no Order/request - OR - Even if it remembers the other item is "This" - Their request is already "completed" and they no longer can "update" from "waiting" even if they can verify they have the other item.
Either way, They become stuck, indefinitely, because the order is gone, there is no timer, and no X, they can't accept items, because there's either no items that fit their order, which is to say, no order, -- or -- They already have both items, and know they've already accepted it, so they can't take all your iron swords in a single trip to the shop and only pay for one kinda logic.
Anywho. Point being.
If you have only for example
Copper and no Tin.
Sell them the shield - and then dismiss them for the profit of the shield - which is the most profit you can earn "on tier", I believe.
If you have for example - an Alloy hammer, and they request for example, Mithril, but you yourself only own 1-3 adelite ingots and 7 iron ingots - Then if you have enough to give them an Adelite version of the weapon they requested - you'll get more money, so give them that instead, and then dismiss them.
If they instead request "Mithril greathammer - with a (quality) grip...." - Just Dismiss them. unless You can give whatever weapon they're requesting. - As for the Shield, I dunno, You might still be able to give them a shield, but It'd probably be easier to just dismiss them.