Usually when I ship a package internationally, the post office has a small plastic envelope that I can use to put three copies of the shipping label/customs declaration form into on the outside of the package. I guess they didn't have any of these envelopes today, so the clerk told me to tape all three copies directly onto the package. Is this allowed? Or should I buy some plastic envelopes to put the forms in? I was reading online that it should be in a plastic sleeve. I just want to avoid any trouble if I get a different clerk when I go back to drop it off. Thank you!