Trademark of a Traitor by Kathleeen Moore Knight, 1943

dustjackets.com

 Nick Duane and Martha Cleves are Americans in Panama. They have been talking about getting married, but Martha feels that their opinions on the war are too different. Martha feels that she needs to do something for the war effort and is returning to the States to do so. Nick rather stay out of the war and has been ordered by his father to the bank in Mexico City.

But then they find themselves in the middle of wartime intrigue. There's a mysterious package that they come into possession of and people are getting killed for it. They get cryptic instructions by a dying man on where to take and who to give it to. Martha is determined to take it, so Nick has no choice but to go along with her to keep her safe. They encounter danger at every turn. They can't be sure who to trust. Who is a Nazi agent and who is with the Allies? Will they get the item to the right person before it's too late? (Goodreads description)