What do you do if the customer delays the signature on the design document but agrees on it Verbally?

I would send my sales representative to have it in writing. Or I will send him official letter or email for follow up and then2 reminders including note of holding him the responsibility of delays in case

3- ask him to send an initial approval by email

Request a Letter of Intent or approval in any written form from the client or top management.

Service Delivery Managers whose projects I manage - already know that I need to see a written 'go ahead' from an executive or a signed PDW or contract.

I still continue in the background with relevant preparation work.