I've been playing the long game with a few series, and that paid off for me recently. A few authors have listed several of their short stories on Amazon in two ways: separately, usually for $2.99 each, and in collected volumes of around two dozen for $9.99 each. These authors often use freebie promotions to highlight different installments in those series, usually one per series per week.
A few days ago, I completed two such series and manually reconstituted the omnibus editions of them. In doing so, I removed quite a bit of duplicated content and streamlined the formatting, yielding what I personally think are more attractive, user-friendlier, and less bulky compilations.
The best part - zero dollars spent.

(I could finish a third such set by paying a buck for the one missing story, but I haven't convinced myself to pull that trigger yet.)