Why letters?

I created this business as a way to not only give back to USPS as a whole, but to thank the people who are carrying out their mission directly. While a lot of people suggest that we buy more stamps in support, there are a lot of people who probably don't end up actually using those stamps or have anyone to use them on. But we typically see the mail carrier regularly (or at least they stop by regularly even if we're not there), so why not leave a letter for that mail carrier while buying a stamp at the same time? All of the supplies that I put into the letters and packaging I will be providing. All I ask is that you pay the cost of the stamp and the PayPal fee that will be charged in order for me to run this website and allow the transactions to run through.