Flattening Wavy Sheets

Can wavy, undulating, or warped sheets in the original USPS shrinkwrapped packaging be successfully flattened in a sheet protector or will the sheet always show some sign of not laying flat?

Below is an example of two types of wavy sheets. I have more late 1990's stamps sheets like this in original packaging. The waviness and curls take up more space in an envelope vs a flat sheet making them difficult and costlier to mail as is. Not sure whether to use as postage, or remove from original package and flatten them out in a sheet protector and then sell that way.

They will not flatten in the original packaging. If you take them out they will probably be able to be flattened as long as the waves aren't real close together.

The reason for the warping is the plastic wrapper that has shrunk over time. And initially, the sheets were wrapped rather tightly by the manufacturer right from the start. At some point, something has to give way and that is the cardboard stiffener and/or the stamp sheet inside the plastic wrapper.

You have two choices, the simple one is simply remove the plastic wrapper and your stamp sheets will likely return to flatness.
The other choice is more daring by applying heat to the plastic wrapper which will relax the plastic and give you back the space needed. A hair dryer, for example, would do the job. The problem here is the danger of locally melting the plastic wrapper if too much heat is applied.

I don't think removing the plastic will somehow diminish the value of the stamp sheets. I have a collection of Commemorative panels which came wrapped in plastic wrappers. The wrappers had a sticker that says "Non-archival plastic - Remove the plastic after receiving the panels" (which was a bad idea but that is another story). Back in the days, still wrapped panels were priced higher than unwrapped panels. I don't think that applies to common sheets.

I still have some like that but not close enough to grab and look at but I believe there was even a warning on the packaging stating it was not meant for long term storage or something like that.

I would think if you took them out, they'd flatten right out

you might want to look on the original packaging. I believe it warns that it is not suitable for storage. That said, remove the plastic, keep the original aside contents, place between two pieces of cardboard or card stock, place a pile of catalogs on top on a flat level surface & after time it should flatten. (Even placing in a correct size mount & into an album should resolve this 'curl' issue,)

Out of the packaging the sheets (panes) should flatten in a standard mint sheet file.

Edit: If you read the last (bottom) line of text on the back of the 50's, you will see, "Packaging not suitable for philatelic archiving."

This is for two reasons, the card stock in these things is not neutral (likely acidic -- bad news) and the clear plastic is a type of shrink wrap or other plastic than will shrink in heat causing warping as similar constrictions.