For sending from US to Canada
If a Phytosanitary Certificate is not needed and not a commercial shipment over a certain lot size then nothing outside a customs declaration.
CITES species would require a CITES permit.
Importing from Canada to the US however requires a Small Seed Lots Permit. If the shipment doesn't qualify as Small Seed Lots then a Phytosanitary Certificate. If over 12 lots an import permit is required in addition to the Phyto. It takes about 15 working days to get the Small Seed Lots permit and copies of the paperwork have to be sent to the sender to be included with the shipment.