Many/most Arduino boards are open source. So if you small and are trying to get a simple product line established you can start with designing a daughter/motherboard that the Arduino board plugs into for first series. Then, once you scale up, embed the Arduino parts into the same board. You can use a clone also, which can be much cheaper.

But I wouldn't go recommend Atmega328 for new designs unless its extremely simple, so when I say "Arduino" I mean something similar, like a Teensy 3+ or Olimex STM32 or Adafruit Feather M0