i would of turned round as soon as i noticed this and took the van back to them sound to me like the box is fooked also shody workmanship take it back and get them to sort it or get cash back before you hand over cash get in the van and give it a little check most places wont want you to drive off before paying so get in start it up move backwards and forwards a little
even better ask them to get in while you take it round the block
hope you get it sorted m8