Currently Estimated Delivery Times shown with shipping quotes is based on a static number that we set in the shipping configuration but UPS / USPS / FEDEX all offer live estimated times in transit from our location to the customer's address.
We also have to set the lead time as a static value for all products shipping via a specific shipping service.  Most of our parts ship the next day, but we also have a wrecking yard where we pull parts from donor cars. If the parts ordered are parts that are pulled off a donor car, those parts can take up to a week to ship, especially if they need crated as well. 
So for example, currently we have to set the lead time for Priority Mail to 2 - 10 days (1-7 LEAD TIME + 1-3 FOR DELIVERY).  If we could set lead time overrides on a product level, and the system could retrieve the days in transit quoted by the service providers, we could provide much more accurate estimated delivery windows for customers.  We have started getting a lot of flack from customers because our estimated delivery dates show something like:
US Priority Mail - Estimated Delivery: Wednesday June 29 - Tuesday July 12
Whereas for most items the system could just add 1 day lead time to the estimate time in transit returned by USPS and say something like:
US Priority Mail - Estimated Delivery: Wednesday June 29