Call your regional issuing office and find out the exact physical address and phone number of the person processing your application. If possible, get a person's name. In your overnight (FedEx, UPS, etc.) package, include the payment, application(s) and a pre-paid and addressed overnight package for the return of the newly issued passports. If possible, find out the easiest/preferred overnight shipping method of the regional office and use that carrier for the return package. Be mindful of weekends.
I sent mine out on a Monday and received the new passports on Wednesday. If your carrier has the option of "first thing in the morning delivery", and the regional office will have someone there to receive the package at that time, pay the extra and select that option.