AbleCommerce Shopping Cart Software

UPS has a XML Time in Transit Tool. That will give you the delivery date of a package if you feed it a shipping date.
This would be a vary good tool if you have multiple shipping options like USPS and FedEx. Then the use can see the prices and delivery dates.

FedEx moves packages on Saturday. And makes home deliveries on Saturday. So to some zip codes FedEx is a hole business day faster. So if the package ships on Thursday FedEx ground could get the package to the customer 2 days faster then UPS ground for some customers.

USPS has a web tool http://www.usps.com/webtools/delivery.htm
For transit time
UPS and Fedex also have transit time tools

Instead of a drop down box for shipping selection if it used radio buttons and sorted by price. And also displayed transit time.