"The company had to send a staff member to each customer to explain the situation."
I think this sentence implies that a staff member visited one company, another staff member visited a different company, etc.
How do you say in the case the company had to send one staff member and that staff member visited all the customers?
The company sent one of its staff members to visit every customer. (That must have taken a while.)
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.