I lived right on the border (Canadian Side) for 55 years. I crossed a couple of times a week and took my daughter on dozens of vacations. Generally, the letter was not always necessary. Rarely was I asked although I got a letter even when I WAS married when I took the kids to Mexico. STILL...
I did get a letter for trips outside of North America. AND...
The border is tougher all the time. Although yesterday when I crossed - the customs officer was far more interested in my car than in anything else. Oh - and that I had Golfish crackers - don't ask.
I thought everyone needed a passport or enhanced licence to get into Canada. I have American friends who can't cross at this time. Not just for flying. But as you already have them - it's not an issue.
