WASHINGTON—In the before times, Katie McCarron could count on her best Canadian customers to make the trip to her store in Portland, Oregon, to stock up on their favourite high−quality, human−grade pet food. COVID−19 had other plans. Soon enough, though, so did Portland Pet Food Co. “Some of them would just be shopping in Portland, and we’d hear that they had been here, or they’d write us and they’d be asking, ’How can I order your food online with the border closed?” the B.C.−born McCarron said in a recent interview. In the United States, however, every international shipment of pet food products requires a special  health certificate, making it impossible for a small retailer like Portland Pet Food to offer online sales outside of the country. “We can’t ship to Canada — it’s just too costly, and we do have to get these certificates issued each time we ship. So …