Is it safe to remove/trim jack studs to make a door opening wider?
For high priced custom doors, you deserve to get it right. 

The king studs are the primary support on the sides of the door (they go all the way up to the top plate.  However, the jack studs are supporting the header over the door and that is supporting the cripple studs over the header.  If this is a load bearing wall, these are all supporting members.



If the jack studs are cut down, they probably need to be cut more than 3/4 inch total to accomodate shims to square the door in the opening. If you take out only 1/2 inch on each side, you have reduced the jack stud by 33%. If this is done in place, you are also stressing the remaining structure of studs, header, etc. in sawing out the excess.

It could be that the overall framing is overengineered enough to survive this assualt, but it is risky, violates most building codes and is a bad way to start a good project.

I would insist that they measure thrice (three times instead of the recommended two) and bring a correctly sized door.

P.S. I had a similar problem with a custom tempered glass shower door that I had waited eight weeks for. They played with cludging it for about a minute and then took it back for a correctly sized piece. It hurts but it is worth it.