Return of faulty item

SuperSpot

Hi, I'm a newbie! Just joined as I have a question about returning a faulty item.
We bought a dog therapy mat online which was described as providing a "highly stable platform for the dog, aiding confidence building, while avoiding any risk of tripping or unsteadiness".
On receiving the mat it was full of air and ballooned out when kneeling on it.
We voiced concerns and were told it was "something that happens about 10% of the time and is due to a rapid change in atmospheric pressure" and to "!!!!!! a couple of the corner seams with a pin to allow the air to equalise more readily".
The mat is waterproof and we were told doing this would not compromise that, but we are not convinced.
This is obviously a quality issue that needs addressing, and the mats in this condition shouldn't be sent out until they are stable and safe to use.
They have agreed to us returning the mat but said we have to pay the return costs as there is nothing wrong with it.
We are also returning a dog bed to them (part of the same order) which after using once caused a massive arthritis flare-up in our dog. The quality of both items is disappointing to say the least.
We are obviously happy to pay the return cost of the bed as it is not faulty, just not suitable. Although we have said we will only pay reasonable costs, and no more than it cost to be delivered (they insist on organising and paying for the courier and will then deduct the amount off our refund).
But are we obliged to pay for the return of the mat?
Sorry for waffling! Just trying to get as many of the facts down.
Any advice would be very much appreciated.

Sandtree

How much extra is the cost of mat adding to the return of the bed? Would have thought they fit in the same box and with most couriers working on banded weights, you;d be unlucky if it increases the cost. 

SuperSpot

They've asked for them to be packed and returned exactly how we received them; folded and in separate bags. 
Apparently they won't know a cost until the items arrive back with them and they get an invoice off the courier.

cx6

In my opinion the items are not faulty, just poor quality. You have the right to return them for a refund as you bought them at distance, but normally you pay the return postage.

It is good they are organising the courier as then the return is complete once you hand the package over to the courier.

SuperSpot

The bed is poor quality.
The mat is definitely faulty. They've said it's something that happens 10% of the time which shows they are aware there is an issue.
The mat is still ballooning when you get on it so very unsafe and definitely not suitable for a 12 year old arthritic dog who is unsteady on his feet to get on.
The courier collection is the worry, as they can just deduct whatever they like from our refund and we won't have any say.