Here is a link to USPS Customs - restrictions and prohibitions
for sending international shipments and mailings.
Here is a link Directly to Canada's Restrictions
Oleomargarine and other butter substitutes, including altered or renovated butter.
Perishable infectious biological substances.
Perishable noninfectious biological substances.
Plumage and skins of wild birds.
Used or secondhand hives or bee supplies.
Precious stones, set or not set; all items used as dress ornaments and coming under the term “jewelry” including articles of gold or other precious metal for personal use such as cigarette holders, cases, powder cases, card cases, opera glasses, fountain pens, watches, etc., are permitted in insured parcels provided the articles have value not over $5 U.S. A parcel containing a number of such articles valued at $5 or less may be insured for the total value of the contents up to a maximum of $200.
Did not say anything about plant material or catnip. I would just be sure to MARK IT ON THE CUSTOMS FORM CLEARLY!! Put CATNIP CAT TOY! or whatnot. Don't suprise them and make them wonder!