Fyndiq define a Missing Shipment as "A package the customer feels they have not received". A customer can only report an item Missing once the delivery window has expired entirely. Therefore it is important you have an accurate delivery window for your articles, to ensure customers cannot report the items Missing.
Missing Shipment flowchart
Customer reporting a Missing Shipment within the delivery window
When a customer reports a Missing Shipment, an automatic check is performed for the delivery time. If the delivery window is not yet expired, the customer is prevented from creating an errand.
I have a valid tracking number
You can only decline an errand regarding a Missing Shipment if you have a fully traceable tracking number* which shows the article has been fetched, or is available at a pickup location near the customer. Otherwise the customer should be offered a new shipment, preferably with a valid tracking number attached in the email, or be refunded.
If the tracking number is valid, share the tracking link to the customer and put the errand's status as to Errand rejected in Reclaimit.
*Please note Varubrev is not approved as fully traceable. Link: Shipping with Varubrev
Customer address is wrong
If you suspect the customer has provided an erroneous address, verify the address by asking the customer in their errand. When the customer replies with the correct address, you can send a new item to there.
Make sure your shipments include your return address. That way the first, incorrect, shipment will be returned to you so that it can be re-sold.