The insurance industry has certain fixed standards for repairs so the likelihood is that the quote from the bodyshop includes replacement of the entire rear quarter panel - it's not uncommon for a non-insurance quotation to be cheaper, though we would prefer not to comment on the ethical element of this. If you haven't already accepted the insurer's decision you have the right to get the van repaired anywhere, so you could get them to cover the cost of repair without categorisation. If the Category N marker has already been applied then the best option for you would be to negotiate the right to buy the van back from the insurer out of your settlement and then get it repaired yourself at the lower price, but bear in mind that it will always carry a Category N marker that will affect its resale value.