What kind of backpack and how many liters is it? Osprey makes a great rain cover (I believe they market it as air cover) that I swear by and that perfectly protects both my 55 and 75 liter backpacks when checking them and also allows you to use your straps when needed without fully taking the cover off.

So they can call it "oversize" (and charge you) even if it isn't oversize, because of the straps and such ?

I think they more mean 'awkward size'. When you put it in a bag, it's not exactly going to have handles on multiple sides like regular cases do, thus making it harder for folks to load/unload.

Awkward would be the best description for the bag. Some baggage systems have issues with it hence the bin.

Nah, they won't charge you but it might end up getting routed to the oversized baggage delivery area

I'm sidetracking from the thread but I have an Osprey Farpoint 40L backpack that has held up extremely well and has a zippered cover for the shoulder straps. Instantly turns from backpack to checked luggage. Highly recommend an investment in a backpack like that for OP's needs. I guess Osprey makes good stuff.

Ha, I have a farpoint 40 from like 2011 and it’s still holding up great and I love it. So much so over the years I ended up buying a farpoint trek 55 and an aether 75. They truly are the best backpacks imho.