When level is a requirment for the use of something, it should give some bonus for the under-leveled item.
It is mainly for spaceship, as we may finally prefer a lower lvl one, but feel cheated as our too great level is useless.
The bonus should make a level X item becoming like a level X+0.33, so if you buy 3 levels "for nothing", they still like add 1 level for your prefered item, instead of nothing at all.