Oh my, they're gorgeous!
Just curious: I usually get my assets from the Unity Asset Store, which doesn't have any Attribution license. Are the paid versions of the assets also under the "CC4.0 Attribution non derivatives"?
I have nothing against crediting the author of something so nice - I thought it was unusual to see both the an Attribution License and a price(a donation, but still..) at the same time.
Typically if you pay for something (30dollars is no longer a symbolic price) it's not mandatory to give credit.
That sounded better in my mind :D
Apologies, I hope I didn't offend you, just making some personal considerations :)
Of course the type of licensing is up to you to decide :)