The most important tool in you shopping cart is shipping rates calculator. The most popular shipping service providers are: UPS, USPS, FedEx, DHL etc. You can use only one or all of them. All these providers has online tools witch calculates how much will it cost to ship from your place to customers shipping address.

If you use open source shopping cart like osCommerce or any other. There are a big possibility that they already has integrated these tools. But if you have your own software, there is good tools for you:

PHP: FEDEX SHIPPING RATES CALCULATOR
Free PHP FedEx shipping rates calculatos class by Algirdas Varnagiris

PHP: USPS RATES CALCULATOR
Another class by Algirdas Varnagiris. Free PHP USPS rates calculator.

UPS PHP
Modules written in PHP to help communicate with UPS shipping API

DHL
This class can be used to submit shipment requests to DHL via the Web.