Vinted complaint: Incorrect advice from Vinted

Let me start of by saying that I made a stupid mistake. I mixed up two parcel so both buyers received the wrong parcel. This was discovered after 1 of the buyers received the wrong item. I then contacted Vinted told them what happened and asked them for advice. They told me to instruct the buyer not to collect the parcel so that it would be returned to me automatically. We followed the instructions but I obviously got paid after several days. I asked Vinted how to undo this and make sure the buyer gets her money back. After all, I pay back the amount I received and shipping but I cannot pay pack the buyers protection fee. Vinted now says they cannot do anything since the buyer did not collect the parcel and did not click the ‘something is wrong’ button, they cannot do anything. I find this unbelievable since we followed up their instructions and now they say because of their own instructions that cannot do a thing.