product_is_shippable() expects $vid

... and in parcel.module, we pass it $nid instead. This could be the cause of other problems as it results in hook_productapi with $op 'on payment completion' not being called on the products in the parcel.

It is also possibly the reason some people are having issues with stock levels updating, but without further investigation into their problem I cannot say.

I'd love to have this patch (probably named incorrectly as I didn't have a issue number yet) reviewed and committed. Let me know if you have any questions.