California Debt Collection Attorney Needed

I run a California-based wholesale company and need a licensed California attorney to step in and collect an unpaid debt from an individual buyer who is also located in California. The case is straightforward: the customer signed our invoice acknowledging both receipt of the merchandise and the exact amount owed, yet has made no payments and has not raised any dispute about the goods or the balance due. I can provide the original, signed invoice along with our delivery paperwork and a timeline of follow-up attempts. What I’m looking for now is clear legal action—beginning with a formal demand letter and, if necessary, moving into court proceedings—to secure full payment plus any allowable costs and interest. Please let me know your approach to California collection matters of this size, typical timelines you see for resolution, and which steps you recommend first. If you have recent, relevant courtroom or settlement experience, share that as well so I can gauge the best path forward.