I have no idea why there is a problem with the purchase label button but you would have to purchase the label on ebay for that service. The buyer has paid right? I don't use .com for printing labels so I'm not familiar with the customs forms but on .ca, any information in the listing needed on the customs form is just transferred over automatically. Even if it isn't, all you should need is the item , cost, place of manufacture and perhaps a few other things.
I don't know why that listing isn't showing the gsp but I did just check all of your current listings and they all appear to be set up with the gsp when I set my location as the UK. However, there are some listings (mostly jean listings) that show you are using ebay standard international shipping for locations that the gsp doesn't ship to and some of those listings also show ebay standard to Australia.
Perhaps someone here who uses the gsp can tell you how to block all countries that don't use the gsp?
It's obviously your decision whether or not to cancel this sale but you do need to make sure that your other listings are set up the way you want them to be.
