One option for shipping that saves a bit and can include tracking is to use Paypal shipping - if you use a Flatrate envelope it is really easy -
https://www.paypal.com/shiplabel/create/
On the flat rate envelopes it saves about $1 in shipping. I also save all of the poly envelopes that things come in and reuse them for shipping. I just weigh things and use paypal or take them to the PO. If you use the paper Flat rate envelopes please use additional tape on them. They are much flimsier than they used to be and usually don't make the trip in one piece any longer. The padded flat rate envelopes fare much better, but have to be ordered from the PO.