In the UK there is a shipping portal called ParcelMonkey that is widely used. They offer courier services and now also Royal Mail and Parcelforce and I use them with great success as its cheap and they pick up from any UK address and deliver to any UK address.

ParcelMonkey just announced an API however I have no idea whether this can work with ZenCart. How does one go about finding out whether ZenCart can be integrated with ParcelMonkey (or the other way round as it goes )?