This has already happened, the Ukrainans loaded some regular trucks with drones and sent them deep into Russia

> How do you counter a swarm of these things coming from all directions?

Throw drones back at them. Shoot them with bullets. Anti-drone tech has been advancing as well, albeit mostly outside the U.S.

I’ve been worried about the same thing, but I’m more worried about it being used as a mechanism for deploying chemical weapons.

>> send in the mail to another country in a special box that can open up when it senses that it has arrived at a destination

They are called letter bombs. Ted Kaczynski did much damage with them. Needless to say, direct mail from Kyiv to moscow is not straightforward these days. And sending secret explosives through third country mail services is frowned upon by postal workers. Fedex even charges a premium when shipping killbots overnight.