Returning an on-line purchase - any advice please.

Any ideas for a quick solution to this situation? I unwisely bought a dress which I saw on-line and which looked really good. I checked that the company had a proper web-site and they did so I ordered it. The company is Milford Manchester and they claim to be 'Trusted by Thousands Across the UK' The dress was only 29.95 (reduced from £60) so I wasn't expecting much, but when it arrived, although it did fit it was not suitable for me to wear being 95% Polyester and 5% Spandex. It was described on the website as being made from 'Breathable Fabric: Lightweight material for all-day comfort'. As I only really wear natural fibres wearing this would be like wearing a plastic bag. I got in touch to ask to return it as the website said
'100% Happiness Guarantee
Try them on at home. If they’re not quite right, return them within 14 days for a full refund — no questions asked.'
However, having had several email exchanges with the company I am now told that I can order something else instead and keep the original dress but they will raise an invoice for £9.95 which will give me another dress but save me spending £25 to send the dress back to China! I am not inclined to order anything else as I don't trust the descriptions on their website. They have offered to refund 10% of my original payment (and I still keep the dress which I don't want)
I am not bothered about losing my money (my mistake to order something which I should have guessed would be dodgy) but I think they have misrepresented the situation on their website. I haven't got time to argue the case with the company as we go on holiday very shortly and OH is going for planned surgery when we return