I'm afraid it's a matter of transportation costs. They only ship containers to large resellers (Amazon). Maybe the Belgian market is not lucrative (big) enough to ship to a belgian warehouse.
In the past before Brexit I ordered from the UK via ebay: now after the Brexit I can understand, but Europrint has its head quarters in Poland and and shipping to Germany or Belgium should not make any difference for Amazon or Ebay in terms of shipping, because I could order from the UK as well (before Brexit).
It's in the Ebay shipping notes of Euro-print that they do not ship elsewhere but Germany
There is no other country but Germany you can select.
I remember from years ago that shipments came from Poland with the DPD parcel service; I can imagine that the original seller made other contracts with sellers in other countries, and they want to avoid competition between those. Article location is listed as Görlitz which is on the border to Poland - on the other side of the Oder river.
I ordered a couple of rolls of filament from a company in the UK, I had been using them quite a bit before Brexit, but this time it took 3 weeks for the delivery to arrive at twice the normal charge rate, seems it went to Germany by road fright first and then to Dublin by DHL Air fright.. Brexit has bollocks up everything..
Last weekend I was able to obtain 15 packages (7x A4 (50 sheets) 8x 10x5 (100 sheets)) of the Expertiz 300 gsm PE glossy photo papers from somebody in my own town, who's printer broke down.