Mona, she's right there's no set price. However, I looked at some of my receipts from last year and they were generally the same price.
I think postage went up again, I'm not sure, but the last time I shipped seeds it was $1.95 plus .85 for the delivery confirmation (optional). I use the 4"x8" bubble mailers from U-line. If you plan to ship a lot it's probably cheaper to order a case of 500 for $69 plus around $11 shipping (at least to here). It comes out to about 16 cents per envelope. I'm not sure how much they are if bought locally.