Usually. My group isn't IT, so with almost every purchase we ask for customization. (IT gets custom stuff too, but they also buy a lot more off-the-shelf stuff.) We have disqualified vendors for not being able to customize their product fast enough or well enough. We pay (a lot) for this. While we could write the code ourselves for a lot of these customizations, the vendor can do it faster and it's easier for us to buy the software than increase our headcount to do the work.

You'd be amazed at how much an enterprise is willing to pay for software, if you have the right niche and are going to make life easier for a bunch of people.