I made up another flat box to give it a try.
This came out to 228g and 36 x 22 x 2 cm. I wanted to send it to Sweden. Sending it as an envelope came to 14.80 (1.80 for signature).
Sending it as Small Packets (as I usually would), would come to 0.173kg 32x9x4cm which comes to 9.90. Hmmm, this wont work for Sweden.
Sending it to USA comes to 7.40 (1.80 for signature). Sending it by Small Packet comes to 8.97. So this will barely work for the address I tried in PA and NY.
Sending it to Canada comes to 3.33 (registered for another 8.85). So that works as long as you don’t register it. Sending it by Small Packet comes to 11.43.
So what might be shaping up here is that the flat box may be good for Canada.