It sells 90 percent of its footwear in the United States while shipping the rest to markets around the planet — to Japan, Singapore, Britain and the Czech Republic.

Growth had been poised to accelerate even more rapidly this year, as Xero wrapped up a test with REI, the outdoor clothing and equipment retailer, bringing an order worth $830,000 for the 2020 season.

“It’s by far our biggest order ever,” Ms. Phoenix said.

But the tariffs that hit Sunday could alter the economics of that deal. Xero could presumably try to renegotiate a higher price with REI that would cover the tariff, but Ms. Phoenix has been reluctant to pursue that route given the importance of the relationship. Instead, she has waited and hoped that a deal between Washington and Beijing would end the threat.

When Mr. Trump started the trade war a year ago, Ms. Phoenix and her husband assumed he would leave footwear out of it because China makes 70 percent of the shoes sold in the United States. How could a president seeking re-election next year put a tax on a product needed by everyone with feet?

But in early August, as the trade war intensified, Mr. Trump threatened to affix 10 percent tariffs on Chinese-made footwear. Last week, after China’s announcement of retaliatory tariffs on $75 billion worth of American exports, Mr. Trump took to Twitter to declare that the tariffs would be increased to 15 percent.

Even before those tariffs take effect, Xero is tallying losses from the trade war. Its new line for the spring of 2019 was delayed as American retailers placed a surge of orders from China to try to get ahead of the tariffs, resulting in congestion at ports.