as far as know people can buy Minifantasy assets using PayPal without a problem. Please, could you let me know the error message you get when you try buying the assets?
As for Unity, the whole Minifantasy collection is already being adapted to Unity (including some QOL features like basic rule tiles) but due to the size of the collection, it's taking time. Once it's ready, the Unity versions of every Minifantasy asset pack will be also accessible through Itch.io.