Who's Responsible for Mailbox Key

[Grey Stone]

I recently rented out a single family rental. They have moved in and are asking me about the mailbox key. I told them they would need to go to USPS. However they are upset by this because USPS is charging $85 to give a new lock and key, also saying I must provide them one by law. I live and invest in California and have never given tenants a key to the mailbox. What do you guys think? Is it my responsibility to give the key to the tenant or is that the tenant's responsibility.