There's a difference between buying postage online and printing it out, vs. paying at the post office... the last time they restructured rates, that difference became pretty significant. For a 6 pound box from you to me, it's $3.55 more if you pay at the post office (and a flat rate box is more yet). Even with the "flat rate" boxes, it's nearly $2 more to pay at the post office. I think PayPal has a way to pay postage right from their site -- that's why it's good to have your address on your paypal account, people can just click and ship when you've paid.
If you're not sure if the weight has gone over 6 pounds, pre-paying online for a 7 pound package is probably still cheaper than paying for either 6 or 7 pounds at the post office. Plus, you don't have to stand in line.
So figure out which way you want to do it... or just calculate the postage and invoice us via treemail. (PayPal has an invoice thingy also, but I don't know how it works from the seller's end.)
