Magic has had what are called "modal double-faced cards" before that allow you to cast either side from your hand, but this is the first I know of that also allows you to pay a cost to flip it while the front side is already in play - in this case, you can play Peter Parker for two mana, and then pay four more on a later turn to have him transform
That way, they will be too slow to swing early on
A company called Enday has some more generic-like nine-pocket sleeves that list at a much more budget-friendly price of less than $4 for 25 pages (or $13 for 100)
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Of those, only 10 have ever been certified by the PSA (Professional Sports Authenticator), and of those 10, only one has been certified with a 10 rating, meaning it is in the most pristine mint condition imaginable