Proof of postage, buyer saying not delivered

[shufflebum]

I posted some childs removable wall stickers last week and got proof of postage. My buyer is saying he has had a card from the post office to say he owes them postage of £2.72 for this item. This is more than I paid to post it. I went into my local post office and they said it is possible that the postage label has come off and I just have to refund him the additional charge. Is this correct? If so why bother getting proof of postage? I can't find any info on royal mail's website.

there will be a £1 handling charge on top of what is short - was the postage £1.72?

If the label came off and it is Royal Mail's fault then I would have thought you could claim back from them?

Yes the postage was £1.72, I wondered why it was more. I guess I'm just going to have to ring customer services and resign myself to being on hold for the next hour!