Freezing Buttercream Flowers for later use?
Put them in the freezer on your parchment covered baking sheet for an hour. Get your Tupperware and cover the bottom with a folded paper towel. Then remove the flowers and put them in the Tupperware. Cover the flowers with another paper towel and seal the Tupperware tight.

The paper towels will help keep them container somewhat desiccated so the flowers don't get damaged by condensing ice crystals and freezer burn. I wouldn't leave them in there for too long though, because they will get burned after some time no matter what you do because of the air space in the Tupperware. Use them within a day or two if you can.