So I took a closer look, and it turned out, of course, that the thing was designed to push a piece of soft plastic against the nozzles. I asked the vendor, and yes, it's intended for storing them when not in use, and should be good for a year or so of storage. Which is exactly the sort of performance I need. So not only were they cheap, they came with something that ought to save me a lot of money eventually, and which HP should be providing if they were actually interested in helping their customers.
If anyone else is interested, this is the set I bought, they sell cartridges in other sizes etc.