If the price paid includes delivery, then it’s the seller’s responsibility to sort it out. If that means refunding you while they sort it out from RM, so be it.
Word of warning to all – even with Proof of Postage, RM can be bloody awkward with this. Their maximum compensation for standard services is iirc about £39. I sent a parcel below this value, it ‘never arrived’ (still not sure i trust the recipient on that point) but i refunded him the £15 or so he’d paid me and took it up with the RM. The max they were willing to refund me was the amount of the postage as I ‘had no proof that the parcel was worth what i said it was’
[edit – despite providing photos, links to the classified ad that arranged the sale and emails confirming the negotiated price]