Ask HN: Why does Amazon not allow certain shower heads to be shipped to CA?

Example: https://www.amazon.com/Waterpik-XRO-763-Pressure-Powerpulse-Detachable/dp/B071D7WCT8/ref=sr_1_7?dchild=1&keywords=shower%2Bhead&qid=1600420162&sr=8-7&th=1

I would assume it is because of certification issues and the manufacturer might have skipped that for Canada. That is pretty common because the process can be expensive and maybe the market is too small or too competitive.

Certification issues are actually quite complicated in our new megacorp international trade. The requirement for certifications is often simply ignored by third party vendors and enforcement is difficult.

If by CA you mean California, years ago they implemented strict rules on flow rate of shower heads in an attempt to curb water usage. Many companies added flow restrictors that customers simply removed. Other designs couldn't be sold at all.