You cannot let them know, there;s no method to do this.
Those messages to post will soon stop, but your payout will be held for a minimum of 14 days.
If this request happens again, it's best to, with the buyer's agreement, cancel the order citing Buyer Requests to Cancel, this refunds the buyer, and doesn't affect your seller performance. You then relist as collection only, for the buyer to buy and pay again. This gives them that all important collection code for you to scan, and this is the only proof the item was collected.
Without it, you do leave yourself open, if the buyer is dishonest, for them to open a case for item not received, and you'd have nothing to prove otherwise.
@lilahchamp2014
