Hey there!
Unfortunately, once you open the package, you can't return it.
As for the Triumph, you'll need to complete one of the masterwork objectives. The easiest one is probably the Nightmare Hunt as you can do one on the lowest difficulty.
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Yes, I did the NM Hunt and completed the triumph. It's just a huge bummer that I essentially wasted $15 on an ornament set that I didn't entirely want.