True, but it seems like the max weight for a letter/postcard is 3.5 ounces (~99 grams), also a postcard is about 148x105mm while a microSD is 15x11mm.

I would propose making your postcard out of a 9x9 grid of 1TB microSDs and tape to give you an 81TB postcard that can be delivered overnight at the minimum price while providing 11.25 Gbps given the numbers you used.