In recent years there has been a flurry of bi-lateral trade deals between countries and the emergence of regional trading blocs. For example, the European Union now has over 30 separate international trade agreements including those with countries such as Colombia and South Korea.
Since 2008 all US citizens MUST have a US passport to be allowed to enter the United States. If your aunt travelled to Jamaica with just her US naturalisation certificate, she would not be able to board her plane back to the US. She would have to come to the Embassy to apply for a passport in order to return home. Issuing a new passport takes at least a week, so this could extend her holiday much longer than she had planned. The good news is that she can apply for a US passport before she leaves the United States.
Entry into Canada: Canadian law requires that all persons entering Canada carry both proof of citizenship and proof of identity. A valid U.S. passport, passport card, or NEXUS card satisfies these requirements for U.S. citizens. Children under 16 need only present proof of U.S. citizenship. Entry into the United States: When traveling by air from Canada, U.S. citizens are required by U.S. law to present a U.S. passport book. A few exceptions to this rule and a full list of documents that can be used at land and sea borders are provided on the U.S. Customs and Border Protection (CBP) website.