Seller shipped without paying full postage

[theverdigriskey]

I recieved a card from Royal Mail today saying they can't deliver item as the sender didn't pay full postage. 

I paid £3.20 in P&P costs and I am sure this would have covered the cost for the item if the seller had gone to a Post Office. I'm guessing he stuck a stamp on it and put it in a mail box. Which also probably means he only used minimal packaging, which means it is reasonably likely to have been damaged. Of course that is just supposition.

What is the best way forward? This has never happened before, to me at least 🙂

[theverdigriskey]

I've recieved this message from the seller after I sent him a scan of the card (un-redacted). What do you think? He seems genuine but I can't see anything on the card about me opening a dispute with Royal Mail as he puts it.

Copy and paste of message from seller: Hello again, I’ve just been on the phone to Royal Mail and they’ve said unfortunately without you calling and making a dispute they can’t do anything. However I’m more than willing on my part to pay for the shipping on my part, I’m sorry that this has happened and I appreciate you letting me know as soon as possible.

[theverdigriskey]

He has emailed me to say he has paid the extra shipping. We shall see. He has been pleasent and at the moment I believe him so fingers crossed!