IKEA does too. You can request smaller part you're missing on their website[1]. And if they don't have them available online you can check in with their support, once they shipped one part from two countries away, free of charge (and even thrown an extra one). For bigger parts they sometimes have them in stock at local stores.

[1]https://www.ikea.com/us/en/customer-service/spare-parts/

I tried that the other day when when my kid rebuilt a 3 in 1 set. I couldn't justify 7€ shipping for a 10c part so that the baby orca could have it's dorsal fin. My kid didn't care. I was disappointed.

Hmmm, so if I wanted to assemble the lovely Cloud City, all I would need is 697 of my best friends to call in and report that they had lost a different piece...