I am happy to confirm that Seeedstudio now have adjusted their PCB shipping fees. Special thanks to Fusion PCB manager Raymond who followed up on my requests!
These numbers are from my own buying experience, no guarantees given on their future validity:
10pcs 5x5cm (1.6mm) was estimated to 140g and $4.93 shipping cost (to Europe).
10pcs 5x10cm (1.6mm) was estimated to 230g and $6.39 shipping cost (to Europe).