Arkansas to Mass, Fender Box, Yamaha bass, Postal service, delivery confirmation, standard mail (not priority, they charge extra for that) - 6 days, $21.38
Use the shipping estimator of an actual shipper (or several - I've had FedEx ground beat the post office), point it at the farthest corner of the continent, and build it into your price. If the buyer is closer, it's a few bucks in your pocket. If they are not, there are no unexpected costs on either part.
I avoid "actual shipping may vary" having been burned badly (one time) in the old days by a seller charging outrageous "shipping" fees for a small item - several times the item price and actual shipping costs combined. If the shipping seems low, I figure the buyer has built a cushion into the base price. If the shipping is absurdly high I figure the seller is looking for rubes that miss the shipping and has built their base price into the "shipping."
Yamaha RBX270 (mfd. by kaohsiung Yamaha Ltd, Taiwan) D'Addario EBT92s, Peavey TNT130
Blessing Artist metal pneumatic piccolo fretless bass (that's a trombone, folks ;-)