I am a senior citizen looking to make some extra income with books and clothes I have to sell. The issue is - I am clueless about how to sell as far as packaging with the post office. Do I pre purchase priority mail boxes from the post office window or can I order them online? Do I print out labels at home and stick em on? What if the item is tiny enough to put in envelope? How do I provide tracking for something that small?

I know mayybe it sounds silly, but I am inexperienced and a senior. Thank you.