Buyer saying there’s a problem after sale completed

Hi, would like some advice as I’m new to Vinted. I sold some trousers, that were listed as very good condition. I sent them to the buyer, who said everything is ok and the money was released.

The next day, she messaged me to say there’s a very small stain on them. I have photos from selling of where she now says there’s a stain, and there is no stain in my photos and I wasn’t aware of one. So I don’t know if she stained them, or I missed it, but I felt bad because it was one of my first sales and maybe I missed it.

I read through all the Vinted policies and told her I can’t arrange a refund through Vinted because the sale has been completed. I offered to refund her if she sends them back to me, which I realise I’m not obligated to do but new seller, trying to be nice etc. (Should I have just ignored her?). Or that she may want to sell them herself if the stain doesn’t come out and she doesn’t want to keep them, as I only sold them for £8 and she’d have to pay for return postage. She decided she definitely wanted to return them, which she has done, and I’ve offered a refund via PayPal. She then said she’d rather have another item I had listed on Vinted (for twice the price) AND expected me to pay for postage because she has paid for the return postage, which I’ve pointed out is in the terms and conditions as her cost, and that I’d be losing out on money to swap them so would not be paying for postage on top. I suggested she reach out to customer service for a resolution if she isn’t happy with my suggestions, but she keeps messaging asking me for the other item and repeating she won’t pay for postage.

So I reached out to customer service myself to see if they can refund her, who have said they can’t because the sale is completed. I’m not sure what else I can do, shall I just block her and move on?