It doesn't matter really what you want them for, the fact is, if you buy something that is advertised as 90 degrees and that is critical to their use, then near enough is not good enough & you should have every right to money back.

You'd be surprised how many people are very hazy about angles. You'd think staff at Bunnings would have a clear view in their head what 45, 90, 180 all look like but you can't assume anything. You'll possibly get a better response just calling it a right angle and saying the brackets are warped and a bit crooked.