Happy to see you posting here now as, funny enough, yesterday I was thinking that I missed seeing your posts for now suddenly to see your question here
Hope you're still enjoying your holidays?
Your question can - partly - be answered by looking at my other Board: the Royal Reading MB
where you'll find direct links to royal bookshops (Van Hoogstraten/The Hague) on top of the Board!
Van Hoogstraten has also been mentioned earlier on here on this Board when they had some lovely royal wedding books for sale
(just to keep this reply on topic you know ....)
I would also like to suggest that you ask the same question on the Royal Reading MB as well, since we've several posters from Belgium / Luxembourg and the Netherlands who all would love to help you out !!
As for the postcards, please, contact me by @mail !!