The US Library of Congress is the only official place to file copyrights. Before any infringement case can be taken to a US court of law, a copyright must be filed. Look at their website.... I think you may now be able to do the entire process on line. It used to be $35 bucks per song and I think $45 for a collection. I used to do collections since it saved money.
Unless this has changed in the last two years the cost for a collection is $35, at least that is what it cost us two years ago.
Also the Library of Congress will take MP3s now. In the old days they only took manuscript! The only limit on the number of songs in a collection is their upload limit. We did around 12-15 songs at a time for the single cost of $35.
{edit} We do not copyright anything now either!