If you choose to pay this extra charge, which Royal Mail impose if items are sent with insufficient postage, then you can ask the seller to reimburse you as you have done.
If the seller doesn't refund you, there is not anything you can do.
If they don't send a refund, then I think your star rating and feedback should reflect the issue you had.
If this happens again. you have the option of not paying and opening an item not received case instead.
"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)
