Buyer claiming item not received after one day past due date, is ignoring messages and opened case

[cassiopeiauk]

I sold an item on 12/4 and posted two days later on 14/4 via second class standard. The item was a low cost dress and so it would have cost more than the bid price to send signed for. The buyer claimed not receieved on 17/4 and i said i posted 2nd class which can take up to 5 working days. 

I received a message today asking for tracking details (a very rude, short message i must add) and i explained that i have no tracking details due to the method of postage and that there have been slight delays due to the current covid-19 situation but if the item has not arrived by friday i would happily refund them, i would just like them to wait a few more days to see if the item shows up. They have ignored my messge and instead opened an item not received case. 

What can i do in this situation, i feel like i have been more than polite and requesting them to wait a few more days to see if there was a delivery delay with royal mail is a valid and responable request? The item could very well show up in the next day or two, as today is only one day after the 3-5 second class mail expectation. It was a low cost item and i don't want to recieve negative feedback because of this, but i feel like its unacceptable that the buyer cannot wait an extra few days

[roger_roger_over_and_nowt]

If you sent by RM and the parcel weighed over 100 grammes and was over 1 inch thick you will have a 16 digit number on your postage receipt.

You can use that to check RM's track and trace service online.