These issues are discussed every couple months here on the website.

I'll just add that tracking is not available for letter-sized mail unless it is sent via priority mail (expensive) or using package/parcel rates.

Try not to re-use priority mail packaging materials. Despite their efforts to encourage recycling, if they find you have used their packaging they will charge you (or your buyer) the priority mail rate. People have fought this, and I have not heard of anyone winning yet. If it says "priority mail" even on the inside of a box and the post office figures this out, they will try to get that rate from you.