The licence costs are always trivial compared to the department's revenue and budget, and it completely avoids any hashing over "will we violate copyleft if we do X". Now the answer is always "we bought it, its ours".
At home, I just use the copyleft version. My personal blog engine is open source, and on github Anything else I make, I don't mind open sourcing if it comes to it.
700 Euro a year is trivial. We've spent more money on drinks while talking about licensing our own product.
Now the answer is always "we bought it, its ours".
If the licence is more restrictive than that, then it is likely the business will fail because if we have to jump through hoops, we might as well (a) use another product, cost isn't a factor or (b) use the copyleft licence and hire lawyers.
You can, as much as you can use any copyleft license - ie. you have to make the source code of the files available. One thing to keep in mind though is that in EU the distinction between strong and weak copyleft (ie. GPL vs LGPL) probably doesn't exist - it hasn't been explicitly proven in court, but there is a past case which shows that the most likely interpretation is that there isn't such a distinction and in EU merely linking does not create a derivative work (you can read about this in this page from the official EC EUPL site https://joinup.ec.europa.eu/community/eupl/news/why-viral-li... which also explains some issues the current wordings have and why the case is not 100% clear).
You can read more about EUPL and how it can be used here: http://www.ifosslr.org/ifosslr/article/view/91/164
The optimistic assumption is that no, that would never happen, your product will not be a derived work because using the library with it is "mere aggregation." But the license doens't clearly spell out what is a dervied work and what is not-- there is not even the discussion of mere aggregation that the GPL has.
The license also has the "death penalty" clause that the GPL does, which means that your license to use it can be permanently terminated if you are found to have violated any part of it.
So basically, I would say, stay away. And people in open source projects should stay away too, for the same reasons... you don't want this fighting it out with the existing open source license of your project.