US copyrights are also free.
However . . .
If you pay to register your copyright, you are entitled to automatic damages ($10,000 per, I believe), per instance of infringement. For example, if the Harry Potter case had not turned out to be total fraud, every move ticket, every book sold -- essentially, every receipt -- would have been an instance.
If you don't pay, you have a longer legal process per infringement, and you must seek specific damages in court for each infringement (in other words, you have to conclude the cost of your copyright, and then convince a judge).
Kanga2 -- Mailing to yourself does not count as proof. It is very possible to tamper with, and no good judge would let that stand.
. . .
We can't stop here! This is bat country!