Where I live, self-checkouts almost universally have a "Use my own bag" button, that prompts you to place your empty bag on the scale and then tares it after you press the button to continue.
I noticed it once when I paid for pharmacy items at Walmart and later put that bag into the bagging area when I bought grocery items. It flagged it as an unscanned item and notified the attending associate.
All in all, it's a superior experience to waiting in regular lines or having to talk to people. Ultimately, I want to just scan and pay with my phone without the kiosk bottleneck.