Hi @edwardsmark,
Thanks for the kind words, glad you’re enjoying WooCommerce!
I can’t speak to the tax rules themselves, so it’s worth confirming with the California tax authority or your accountant what applies to your products and at what rate. On the WooCommerce Core side though, charging tax only to California customers is a matter of settings, no code needed:
1. Go to WooCommerce > Settings > General and make sure “Enable tax rates and calculations” is checked.
2. Go to WooCommerce > Settings > Tax. Since downloads don’t have a shipping address, set “Calculate tax based on” to “Customer billing address”.
3. If only your digital products should be taxed, add a new tax class (for example “Digital”) under “Additional tax classes”, then set that tax class on your downloadable products.
4. Open that tax class’s rate table and add a rate with Country code US, State code CA and the rate you need. Leave the other fields blank so it applies to all of California.
Customers with a California billing address will then be charged that rate on those products, and everyone else won’t.
This guide covers each of those settings in more detail: https://woocommerce.com/document/setting-up-taxes-in-woocommerce/
I hope that helps!