A single first-class stamp will send envelopes up to 1/4-inch thick, so long as the envelope has uniform thickness (no lumpy seeds), up to one ounce in weight. If your envelope weighs more than one ounce, add $0.15 for each additional ounce.
If the envelope is lumpy but still under 1/4-inch thick, you can send it as a non-machinable envelope, for $0.17 extra postage (the post office also sells non-machinable forever stamps).
If your seeds are bulky (i.e. beans) and the envelope is more than 1/4-inch thick, or if it weighs more than 3.5 oz, it will need to be sent as a package, which in my experience usually costs between 3-5 dollars.*