Read Thread: Animal deterant?
You can make a similar concoction at home for very, very cheap:
Take three raw eggs and purée them thoroughly in a blender or with a high-speed food processor. You need it as liquidy as possible - no solid chunks.
Take a rinsed 1-gallon milk jug and pour the eggs into it. Add 1/4c red pepper sauce (in the Tabasco section at the grocery store). Fill the jug to the top w/ water and shake well. Get a garden-grade spray bottle (~$1.50 for one made of recycled plastic at WalMart) and fill it with the mixture, and you're good to go. You can keep both the spray bottle and the jug in the fridge for weeks/months at a time.
How it works is the eggs in the mixture will putrefy as they dry on anything you spray. It's not enough to bother humans, but it should deter most animals. If they do manage to get past the smell, the pepper sauce will drive them away after one bite.
I use it extensively in our yard and garden to ward off the deer. I have tomatoes, peas, beans and CORN in an un-fenced garden and nothing gets eaten. You do have to re-spray after a few weeks, or after a rain shower, but it's a cheap and eco-friendly solution. I would think that a semi-sheltered letterbox would keep the scent for quite a while. I swear by the stuff. My mom uses the Liquid Fence product with mixed results - it smells awful to humans and doesn't seem to last as long.
You think if I use this near my trash can it will keep animals from tearing my trash up? What about unwanted guests? Hmmm I think this is opening or should I say closing all sorts of doors for me.
I work in the landscape business and I do not think that Liquid fence is going to work on any animal but a rabbit or a deer. I know the Liquid fence for dogs and cats does not work. I have used both of these products and have had success with the deer one. No luck with the cat and dog. Some people believe in hot pepper wax, coyote urine, deer solutions and bobbex. Most people have determined that the more it stinks the better it will work. Just my two cents