You can certainly offer ' Free Post' but as we wll know there's no such thing and offering Free Postage simply means the seller has incorporated the amount of the postage cost, into the listing price.
I haven't checked, but I would imagine such a weight 25 kilos, would incur a considerable courier charge, even more so to Bulgaria, have you checked to see the actual cost? I'm sure you would not like to pay this from your pocket.
As for Custom duty, this is not for you, but for your buyer to pay. Again do they know the cost? It also may be extortionate. If they don't pay it, your item will be stuck in Bulgarian customs, and neither you, nor your buyer may well not see this item again.
Personally, the cost of postage, size and custom difficulty in Bulgaria, would put me off selling this item to this buyer.
I would aplogise and politely decline, but of course, the choice is yours.
