But see, it’s possible to put your foot down without “making a scene”. The trick is, smile. Lots. I’m not a brazen, inconsiderate, or rude person, but no matter who the bride was and no matter how tender her sensitivities, I would have staked my claim on my “stuff” PDQ. With a big smile.
Absolutely.
(A) You paid good money for it.
(B) She didn’t.
So, when Person A, who did not pay for it, takes something away from Person B, who did pay for it, without Person B’s permission, we call that…“stealing”. She was stealing your stuff. You have every right to wade in there and say hey.
And chances are excellent that she’s done this “stealing” thing before, and she’ll do it again. It’s passive-aggressive behavior, manipulative as all get-out. And it’s people who don’t want to “make a scene” that enable it. 
What you do next time is, you say, apologetically and with a rueful smile, while you busily extricate your stuff from the box, “I really spent wayyyy too much on this, so I need to keep it and I hope I get a chance to use it at some other party…” And then you say, as though struck by a brilliant idea, “But–you say you need it for your son’s party? That’s perfect! Why don’t I sell it to you, right here?” And then offer her a price that’s, oh, about 20% off the new price, because, you say, “Well, it is used, but it’s not THAT used…”
I still don’t “get” why the bride should somehow be preserved from finding out that one of her friends is behaving badly. Does her impending marriage confer some sort of sacred status on her, that she needs to be sheltered from icky facts of life? If it had just been a regular party of some sort, and she was there but not the guest of honor, would you have balked at “making a scene” and retrieving your party decorations from her friend? (If the answer to that is “yes”, then IMO you need some assertiveness training. 
)
Oh, and…be sure to mention to the MOH with a light laugh that the party is of course going to involve serious drinking, and that of course she wouldn’t want anyone who was a Designated Driver to have to miss out on the fun, and then ask her, “You don’t mind if some of us stay the night, do you?”
Then tell me what she says.
Okay. Throw one for her yourself. Do not invite the MOH. That will work.
[weary sigh] One of the things I have learned in life is that there is sometimes no way on God’s green earth to understand why two particular people are friends. For all you know, the bride is perfectly aware of her friend’s propensity to punk out, but the relationship offers other rewards to her, and so she overlooks it.
I have also learned that people ask other people to be maids of honor and bridesmaids and whatnot for sometimes the most peculiar reasons. For all you know the MOH has incriminating photos tucked away somewhere and threatened to use them if she didn’t get the coveted MOH position. You’ll just have to let it be a mystery.
But what you DON’T have to do is allow a possibly exaggerated sensitivity to the bride’s feelings affect your behavior to the extent that you’re left feeling like a complete mope. Stand up for yourself; if the bride’s old enough to get married, then she’s old enough to understand that some of her friends might not like some of her other friends. My daughter’s “set” is like that–at any given moment, half of them aren’t speaking to the other half, with her caught in the middle.
Personally, I think that “Sex Toy Party At The Bitch’s House In East Bumblefuck” sounds like a rollicking good time, 'specially if you can get at least one other person to leave her checkbook at home. An hour’s drive for me would be to Chambana, and hey, I’d drive to Chambana for that.
Hon, you’re a caretaker, and I respect that, but sometimes you need to stop being a caretaker and just step back and let the shit hit the fan. You can’t protect your bride friend from finding out sooner or later that her MOH is a lameass cheapskate. Quit trying to make sure everybody else has a good time and just concentrate on your own self, eh?