There's a recent wrinkle if you do int'l shipments, neither ebay/paypal allow buying Express Int'l postage online anymore, it's Priority only now.
I just did an Express Int'l shipment thru Paypal last night.
One thing I had problems with though on that one was printing the label. I use an older Mac (dual G5 PowerPC, running Leopard OSX10.5). The Paypal shipping app (Pitney-Bowes) uses Java (or passes thru to USPS, which uses Java). I have disable Java in my browsers per the US security notice the other day. I tried re-enabling it and still no go, so finally I grabbed my son's Macbook and did it that way.