Actually UPS has the most strict certification requirements that also involve testing of the software by independent testing lab. API is public but if you are going to sell the software that integrates with UPS the certification is mandatory. Fortunately X-Cart is already certified by UPS
http://www.ups.com/content/us/en/bussol/offering/technology/alliances/application_ecommerce.html#X%2DCart+...art+ Software
and LiteCommerce is in the process of certification.
As for the UPS WorldShip we have already implemented the integration with this software but unfortunately it was not included in 4.1 release.
We will include the export to UPS Worldship utility in one of the upcoming versions.