Dallas here from the Shopify Social Care Team.
It is possible that there are fees from the shipping company (UPS) for customs if the items are shipping over the border. I found a link about the brokerage fees for UPS depending on the value of the shipment. This page here goes into more detail on the fees as per the values of the products.
In order to have a further understanding of what this could be in relation to I would need to see a screenshot of the card that she received. Did the customer send a screenshot of that by chance?
If you can send that over I would be happy to see if I can help you even further.
This JUST happened to me also. Order shipped from USA to Canada using UPS (Shopify Shipping). Label printed, along with 3 copies of commercial invoice attached to package. Package is now being held in Canada by UPS for payment of 'brokerage fees, brokerage GST, brokerage QST, Government GST and other government charges'.
I have shipped plenty to Canada and this has NEVER happened. Why now?
I live in Canada. It's just the luck of the draw. Customs randomly selects parcels to be processed for customs and duties. I find 80% of my orders come across without this step. The odd order though is chosen and I have to pay to get it released by the shipper. I am always shocked when that happens. It's not related to where the package comes from, or the shipping company. It's customs, but the shipping company has to collect on their behalf before delivery.
Hope this helps!