What to de re no postage on parcel?

[horsemadgal]

I sent a parcel 1st class recorded delivery on 4th April. Didn't arrive until today buyer has messaged me to say card through her door from postman stating to collect parcel that had no postage on it.
I have the receipt to prove that I did pay for postage, I assume post office didn't attach sticker or something.
Do you know how to claim the money back if buyer has no choice but to pay? I offered to send a scan of the recipt but not sure if her sorting office would accept that.
TIA

Get the phone number from her card for her local post office (or the number for the PO where you bought it from), ring them and play hell. At this stage there is no proof that this is your parcel so they may have to pay it and then claim it back.