I'm guessing plenty of bouldering mats have changed hands over ukc, so what was the best/cheapest way you found of shipping them? I'm lookig to buy someone's mat, but they're claiming it'll be £50-60!! Surely it can't be that much??
60 quid??!!! No chance. I shipped an entire drum kit from 'up north' to Cornwall for 25 quid - considerably heavier and harder to pack! Check other courier services not just Royal Mail/Parcelforce. There's a good one that does pickups from local convenience stores down here, forget the name but it does national deliveries and is pretty reasonable.
Use Parcel2Go, put in the dimensions and weight, and off you go. Tell the seller to use whichever service you like the look of, if they are awkward then tell them that you'll book and pay for the courier yourself. They might have just got a quote for one of the specific oversized parcels (£60 sounds like a pallet-sized delivery), some couriers won't charge that much though of course.