How to safely store melamine plates?
Although there is a known problem with so-called melamine furniture and shelving off gassing of formaldehyde, the source is the particle board within, and not the melamine veneer.

Melamine dinnerware — often known by the genericized brand name, melmac — is by itself quite stable.

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Either of your storage options should work just fine for melmac. Assuming your cupboard has a door that's kept closed, the dishes might not accumulate very much dust. However, you could use a paper, plastic, or cloth towel or bag as a dust cover — or else just rinse off the top dishes before you use them.

If you decide to seal the melmac in a freezer bag, it will not suffocate and should be just fine. Just make sure it's absolutely dry before you seal it up.