Does Ubercart have an external API?
Ubercart does have an API (see http://www.ubercart.org/docs/api and http://api.ubercart.me/ ), but I don't see general REST or SOAP support related to fulfillment.

Some options may include miniRESTapi or as noted UCServices and Bee Colony for Drupal 6. There are some vendor-specific SOAP implementations for CyberSource, but I do not know how well-supported they are.