Simmons used to do that work also. The problem is that the work requires both extreme care and proper equipment. A lot of time goes into making sure the barrels end up pointing in the same direction, not an easy task even with modern laser equipment.
The result is that unless the shotgun is quite valuable, the cost of the work will far exceed the value of the gun. For the average old shotgun, Belgian import, Crescent product, hardware store brand, it is simply not worth the cost of resoldering.
A Parker or a gun of that value, it would be a different story, but high quality guns usually don't come apart in the first place.