Thanks wideman for the info. I can tell you that we attempted to get the customs stamp - but when the office is closed, what do you do? There was a red "hotline" next to the window with a sign saying to call customs if no one was at the window. That's when the customs officer who answered the phone gave the response that I mentioned above.

Note - the box we placed the forms in was marked specifically for VAT/Customs use. It was most definitely not a mail box.