Major changes regarding ammunition purchase take effect in January, 2018 starting with California Penal Code 30312. All ammunition must be purchased or picked up from a CA DOJ approved ammunition vendor. You can still purchase ammunition online, but it must be sent to a local approved vendor who is willing to accept it on your behalf. It's likely that any vendors who are willing to do so will charge a fee that wipes out any potential savings.
Those travelling out of state who purchase/obtain ammunition have to ship it to an approved vendor in California and pick it up there once they are back in California. (California Penal Code 30314)
You can only give ammunition to immediate family members as defined in California Penal Code 16720. That includes spouse, registered domestic partner, parent, grandparent, child and grandchild. If they aren't an immediate family member, the transfer has to go through an approved ammunition vendor.
Starting in July, 2019 ammunition vendors will also be required to send ammunition sales information to the CA DOJ. That information includes your ID, what ammunition and how much you purchased. (California Penal Code 30352) Furthermore you will need to pass a background check (and possibly have a new ammunition purchase ID card) before you can buy ammunition.
These are major changes and doubtless will cause confusion during the initial months they take effect. Don't be caught unawares.