Why can’t I just mail myself a certified letter?
The law does not consider a certified letter as evidence that the contents therein were written on the date of the postmark for a variety of reasons, including that the envelope could have been steamed open and the contents added at any time. Please use either an inventor’s notebook or use the US Patent and Trademark Offices’ document disclosure program to establish your date of invention. Under the law you have to file a patent (in the US) within one year of a public disclosure or offer for sale. Having a certified letter showing that you invented the item will provide you no protection under the law.
Is my invention patentable even though it is a combination of old items?
Every invention is a combination of known items, since you cannot make something from nothing. For an invention to be patentable it must have utility, it must be novel and it must be non-obvious.
Utility means the item or invention must have some practical use.
Novelty means that an examiner cannot find one reference, usually a patent, that shows all the elements and connections between the elements in a claim defining your invention.
Non-Obviousness means that an examiner cannot find two or more references that show all the elements and connections between the elements in a claim defining your invention. Note that the examiner has to have some reason for combining the references, such as they are closely related from a technology point of view or one of the references suggests the combination.