Vinted complaint: Vinted not upholding terms of offers for Buyers

Vinted notified me that I was entitled to free shipping on my first Vinted Purchase and that I should use their “buy now” button for the saving to be applied automatically. This wasn’t upheld and I was charged for the shipping on my first ever purchase. I asked their CS to look into and provide a refund (to the tune of £2.29) which I was told they couldn’t do as the offer related to transactions ie sales and had apparently been applied to a earlier sale of an item – this makes no sense as one) the wording of the offer specifies “first purchase” only and two) the seller doesn’t pay for the shipping fees on sales so the offer would not benefit the person it was directed towards.
I have tried to take this up with Vinted directly whose CS just cancel the chat when they can not explain how this would be the case. I don’t really care about the cost of the shipping but I think out of principle Vinted will be making £1000s by not sticking to their own promotional marketing which is against consumer rights laws in the UK. I’m not a lawyer and would welcome anyone who is to pick this up with them further.