Jules, I think you understand now that it is best to ask all questions regarding any transaction prior to bidding as to do so afterwards can only cause angst to both you and the seller and is unfair to any seller.
Worth considering when benchmarking your idea of what is fair on postage is that the seller may also be charging for insurance and tracking, depending on the method they use.
Whilst I know, from experience, that some European sellers (not including the UK here) can charge what we deem as expensive shipping, it is not always so once the parcel arrives and you see the postage label and take into consideration packaging costs as well.
Also worth noting, a seller pays PayPal fees on the postage element of your payment as well as the PayPal thieves charge a commission on the total amount a seller receives, not just the amount of the actual sale itself.
Of course, if it is not too late, you may ask the seller if they will kindly consider cancelling the transaction altogether. Although I don't think they will be too happy having lost other potential genuine buyers, having to go through the non paying bidder process and then relisting again ... but if you are that unhappy, perhaps the seller will consider this option?
I have sold quite a bit on ebay myself so I do know how the fees etc work, however this is a very small item which according to Royal Mail would only cost £5.66 to send to Belgium by International Airmail Signed For service, and in this I'm allowing for a generous packaging weight. The seller has now sent me a nice little note agreeing that the original €20 charge was too high but has still not adjusted the invoice on Paypal.
Incidently, regarding those Paypal thieves(!) since Paypal is owned by Ebay (at least it still was last time I checked) I think Ebay is having a laugh at our expense, charging transaction fees twice in effect!