Use the Ratman rather than the Overprepper. Not only does the Rat give MORE items, it also gives the items to better targets. Aka. your other units that hit multiple enemies or take reduced damage.
Overprepper would need 4 items to be as effective as the Ratman.
The Couponer works just as well with the Rat. That's the issue here: the Ratman is just always better. Because it triples the effectiveness of each item, instead of doubling it like the Overprepper. Plus it gives the extra items to units that have a better use for it than itself.