US Postal service priority mail flat rate seems to be the best bet, $12.95 plus insurance if you wanted it (I think that runs about $2)
That has a delivery time of 6-10 days but I think usually shows up faster than that
They do have a first class mail envelope thats a little cheaper, about $8 but I'm not sure if I could get all of the tuners into an envelope and still have it seal up around as it would be so lumpy they might not allow it as I guess you have to have the envelopes rather flat and standard