I put all six books of my Daughter of the Wildings series plus two short stories connected to the series in an omnibus. It's not on Amazon (it's priced way over $9.99), but I put it on the other stores (I'm wide with all my books) and regularly submit it to Kobo's 40% off coupon box set sales. That's my big moneymaker on Kobo. Bought a new printer with what I made from it in September. (That's a big deal for me.)
Now, I wouldn't have done that except: 1 - I'm wide, so I could put it on Kobo, where buyers seem to like big box sets; 2 - the whole series had been out for a couple of years and needed a boost; 3 - I've started releasing the follow-up series and also have a decent-sized backlist for readers to dive into. If this series was all the books I had out, I wouldn't put them in a full-series omnibus.
I also have a box set of the first three books. Readers seem to like that because it's a savings over the three books separately, and I like it because sell-through to the other three books from the box set is very high. This one is on Amazon. I rarely put it on sale, and then only for $2.99, though if I ever feel like I can afford to cough up for Bookbub, it'll be my 99 cent Bookbub bait.