I submitted my first commercial Concrete5 add-on for the Marketplace today. It's also the first third party add-on module of the Concrete5 eCommerce package. The module allows you to retrieve shipping rates and service information for UPS shipping worldwide.

I did most of the work on the payment gateways that are part of the base eCommerce package, so the UPS module was not the first module I wrote for the eCommerce package.  I implemented the Paypal integration code for the Paypal's Website Payments Standard API.  I also wrote the gateways for both the Authorize.net Server Integration Method (simplified API) and Advanced Integration Method (for self-hosted sites using SSL).

The UPS module provides a simple configuration interface for entering your UPS account information and for choosing the shipping options you want your site to support.  One the front end of your site, the module is fully integrated into the core eCommerce package.  When it's enabled customers are show any UPS services that are available based on the products they are purchasing and their shipping location.

Please see the UPS Shipping Module product page for more details.