480 000 000 000 000 bits / 86400 sec = 5555.56 mbit/s
And you can send as many boxes as you like in parallel.
I fondly remember conversations on AmiNet - an Amiga - dedicated FTN network.
That was all done over PSTN with 9600 - 19200 bps modems. Latency was days. All this didn't preclude massive amounts of collaboration over it.
I'd rather question if lower latency delivers any benefit.
I imagine that in practice the postal service may start to decline your custom somewhere around the quintillion-box mark, or perhaps even before...