Filing a copyright online is one of the fastest and inexpensive ways to get your work protected in the most convenient way. This is true if you need to file a work in US Copyright office and you are living outside US. See the cost comparison for example in Philippines:
Cost of courier: 2500 pesos
Copyright fee: 2200 pesos
Total: 4700 pesos
Number of days to arrival: around 1 week max (for using express mail)
Converting this to US dollar at 48 pesos per dollar: $98
Expensive. I do a research online of what are the safest and cheapest online copyright options. Online work can be completed with less than 50$. You can save a lot of money if you will a work online. First let me tell you the important things when you are doing copyright application online:
1. Do not trust third party online copyright processors that are not reputable enough. I am not going to recommend these type of providers as there are many of them but you can find some of the trusted ones by doing a Google.com search for a term such as “File Copyright Application Online”.
I have bad experience with these third party online copyright processors, one website guarantees me 2 days processing at more than 100 dollars. The reality is that they complete the work in MORE THAN A WEEK! When I ask for refund, they said they have just submitted the package on that day to US Copyright office (same day I ask for refund) 
Do not believe in guarantees and prepared to risk yourself , your time and your money if you like to avail these third party online copyright processors
2. Take note that these online processors will still have to process your data in digital form, then print it to a hard copy for your documents as well as burn those songs in the CD. And finally send the package to US Copyright office. This takes time and it is why they are expensive.
3. Watch out for Fraud! Do not transact online or submit your work in the area is not secured. You can check if it is secured if you can find the https:// in the browser address bar for their submission page.
In Firefox: https (secured) can be seen on the address bar:
In Internet Explorer, it can also be seen in the address bar plus you can even check the security if it valid by clicking the “golden padlock”.
Make sure the security belongs to the website you are transacting at the very moment NOT to the other website. Feel free to walk away if it does not pass the security test. This is to ensure that any information you are submitting are encrypted.
RECOMMENDATION: The best way to file copyright online can found in US Copyright Website itself. It is cheap, around $35 dollars and you can upload your work DIRECT to the US Copyright office without having to fear of being lost and ignored by third party copyright processors.
Advantages:
1. You can pay using credit card.
2. It is direct to US Copyright office.
3. You do not need a courier anymore.
4. It is secured, and this is the exact URL: Eco Online System (note: It requires you to register and login first)
One disadvantage I have seen is that it will timeout every 30 minutes, so if you have one very large compressed file that will take more than 30 minutes. It will be disrupted (data lost). In this case, US Copyright recommends to break it down into manageable pieces so that it will still be uploaded completely within 30 minutes. Good luck to your filing!