Contact the seller - there is no way to reclaim the monney you paid through Ebay.
Large letters are more likely to attract a fine because of thickness rather than weight. So, sometimes small clothing items sent as large letters 'ruck up' and get stuck in the machines at the sorting office, or sellers don't realise that little, light items may still be parcel size if they are over 25mm thick.
I sometimes have to send small, light pieces of jewellery as a parcel because they have a large central bead or I have included a little box to keep the item safe.
"There are two ways to be fooled. One is to believe what isn't true; the other is to refuse to believe what is true.”
Søren Kierkegaard, Danish philosopher (1813 - 1855)