Actually... some banks and other places have this pneumatic post system... You can cram an entire container full of highest capacity microSD cards and that would also be a lot of data. Compared to a trebuchet, this has the benefit of having a theoretical ability to sustain a decent throughput. And it's two-way, too!

Either way you still have the problem of someone having to put those cards into readers to get data off of them. That'll be the bottleneck of this system, unfortunately.