You can't flash the bootloader over USB, because the bootloader is the basic software that enables you to flash your actual firmware over USB. Teensies come with a preconfigured bootloader, but if you start with a raw micro controller, you have to flash the bootloader over the chip's ISP interface ...
I would be interested in an assembly service as I don't have a soldering iron.
As it's being sold on Massdrop you could ask them about previous drops where kits or fully assembled was an option. I guess the proportion of orders for fully assembled would determine whether it's worth the extra ...