You are likely in the clear. By stopping any infringement the other party may be satidfied with the result. Unless you made a lot of money from sales of the app., or they lost a lot of money from sales of the app., suing you for damages likely will not be in their interest, especially as it sound like you have ceased any infringing activity and it sounds like you had no intent to infringe in the first place.
Still, according to US trademark law (Lanham Act, 15 USC 1117) the other party could seek (1) your profits from any sales using the infringing mark, (2) any damages they sustained by the infringement, and (3) the costs of bringing the action. There are a variety of factors to consider regarding whether that could or would happen—and, as mentioned, it sounds like pursuing that route would not be worthwhile for the other party.
Please note that this information is for informational purposes only and is not legal advice and should not be construed as legal advice. Nothing herein is meant to create an attorney-client relationship.
There are two issues in play: (1) does the name of your app really infringe the trademark rights allegedly owned by the other company and (2) even if it does, are you legally obligated to stop selling the app?
Without knowing the name of your app or the alleged trademark owned by the other company no one can answer either question [and do NOT submit that information to any public forum, including this one]. So ... you cannot receive any actionable advice here.
To the broader question of "can this US company sue us for trademark infringement" the answer is yes. If you do business in the US then you can be sued here -- and selling iPhone apps to customers in the US is doing business here [in addition, the agreement with Apple that you entered into to sell your app requires that you consent to being sued in California (by Apple, at least, and perhaps by everyone)]. If you don't show up in court, the court will issue a "default judgment" against you and the prevailing company will be able to enforce that judgment against you in Canada.
Be warned: many trademark owners who send cease and desist letters are just plain wrong about the scope of their trademark rights. You need to have a trademark attorney in the US review the situation to evaluate whether the name of your app does, in fact, infringe the other company's alleged trademark. Good luck.