I BELIEVE (opinion, not fact) the ship option shows up as long as the customer pays you through paypal indicating it is for "goods", not services, or e-bay or whatever the rest of those choices are.
And, EVERY TIME I go to ONE of our post offices, the form is the wrong one. The other post office just ships it, as is, none have ever come back.
This is yet another one of my "pet peeves": There are two post office buildings within 5 miles of my office - in opposite directions. The one "downtown" is VERY helpful, friendly and things go out fairly easily - but parking is a pain. The other one, where parking is very convenient and there is more staff, so the lines are shorter, employs jerks. They delight in telling me SOMETHING is incorrect. Of course, since all my stuff is over 14 oz, I can't just put it in the mailbox. Lately, we have tried "carrier pickup". That's pretty good - but can be a problem when I really don't KNOW which forms SHOULD be filled in. The carrier certainly doesn't know.
I, too, will be looking for PayPal international, I've always gone to USPS for that - adds several steps, PayPal would be MUCH easier.