In my experience, it will still get there.
I don't know which country you posted from, but many countries have bilateral agreements on trade, so it may not even have been necessary/required to fill out such a form. I certainly never have, evne when posting to the US, Canada or Australia from the Netherlands.
Worst case, customs will open it, slap a fee on it and the receiver will be alerted that they will have to pay import taxes / customs fees before they will receive the package. But most likely, it will just be delivered to the receiver as is...