Don't bother mailing it to yourself--that doesn't work in the US.
What you want to do is open a file with the USPTO. They have a document disclosure program where you file those documents down so you can call upon them at a later date (e.g. deposition).
If you want a patent, don't bother writing one yourself or having an engineer write one unless he has a legal degree as well--the wording of the claims in a patent is crucial and overlooking the slightest detail or failing to follow protocol will cause misery when your patent is invalidated later on.
If you think it is worth it, just spend the money on a credible lawyer. DO NOT go to those patent agencies--those are usually scams (e.g. 1-800-invent).
You basically have your 2 types of patents: design and utility, and of those you have either provisional or non-provisional. You can try writing the patent yourself now and file a provisional patent which will hold the date of filing for up to 1 year, but you will have to file a non-privisional later anyways so why waste the time.
Partners at law firms bill out at 500-600 / hr and associates at 100-300. If can cost up to 25k just to get it to filing, and up to another 50k up to issuance. Be sure you are ready to pony up the cash. Go big or go home.
Good luck, and let me know if you need anymore help. Remember to seek professional advice.
Oh--take your documents and transfer them all to a hardbound lab notebook with the pages numbered, and each page signed and dated. Maybe get it notarized or have a witness sign and date it too. This is crucial to prove you invented it first. Or open the document disclosure program.
Before you try to file, do a search on
http://www.google.com/patents to see if anyone else has patented it (if you trust google--I personally don't). Or you can try searching the USPTO documents, but that is a little more complicated. Lawyers will usually wave the consultation fee, so don't let that put you off.
Good luck.
