It pays to ask. Sometimes priority mail is actually less than parcel post.
Or maybe only a nickel or so more.
I bought an electronic scale that is fairly accurate (maybe to within 1/4 of an ounce) - I weigh what I want to send and then look up the USPS rate calculator to get an approximate cost.
(The scale cost around $30 if I remember correctly - this was purchased during one of my diet phases)
I use it often enough to be glad that I have it. It only goes up to about four pounds, though.