Can I claim interest and costs on an unpaid business invoice in California?

Prejudgment interest, meaning interest that accrues before a judgment is even entered, at a rate of 10% per year is generally available on a contract claim involving a specific, ascertainable dollar amount, calculated from the date that money became due. Attorney fees are a separate matter entirely, those are only recoverable if your underlying contract specifically contains a clause providing for them.

If your standard contract terms currently lack an attorney fees clause, adding one to every contract you use going forward is worth serious consideration, since a fee-shifting provision changes settlement negotiations more meaningfully than almost any other single clause you could include.