No.if I understand correctly, Apples own article says that you pay 15% if eligible, and when you cross 1million, you pay 30% for the remainder of the year, ie no back payments.

That would be dangerous, because imagine being under 1million all year, and in December you make 1 dollar too much - suddenly you would have to pay Apple a lot of money you may have already spent. You never pay Apple though, they just keep their cut.