It seems obvious but...
Alice and Bob have an existing business relationship. Alice sends Bob a letter demanding his response via U.S. Postal Service (USPS) Certified Mail, to be received on or before Tuesday, June 28, 2022 or Bob will loose rights under the business agreement - which, if lost, may also involve future legal proceedings. (Dates are arbitrarily chosen to show timing of events - this is not a request for legal advice, Bob already has a very good lawyer :) ). Bob responds to the demand as requested (demanded?), in good faith, and sends his response via USPS Certified Mail on Thursday, June 23, 2022. When the USPS accepts the Certified Mail and provides proof of mailing, it estimates that the mail will be received by Alice on Saturday, June 25, 2022 (not that it matters, but Alice and Bob are in the same city).
Neither the agreement between Alice and Bob, nor the appropriate section of the California Business & Professions code specify anything more than requiring Alice to set a response date for Bob that must be at least a certain number of days after the demand.
Two closely related questions:
If the Certified Mail is delayed in transit (outside of Bob's reasonable control) has Bob met his requirement under the agreement? If the dispute proceeds to court, is the court likely to forgive Bob for missing the deadline and keep him from loosing his rights under the agreement?
Bob has sent his response using the USPS's Return Receipt feature that requires a signature for proof of delivery. If the USPS attempts delivery on or before the June 28, 2022 deadline and Alice (or her representative) is either not available or will not sign for the mail, has Bob still met his requirement under the agreement? If the dispute proceeds to court, is the court likely to forgive Bob for missing the deadline and keep him from loosing his rights under the agreement?
Specifically looking for how this would be handled in California, but information for other locations is appreciated too.