I was on a cruise ship and when we would go to varying ports we had to present our passport upon disembarking. I bought the Van Cleef bracelets in Tokyo. The cruise ship left Kyoto 3 days later. I was questioned in Kyoto and perhaps one or two other ports. The bracelets totaled some $$$. I doubt pens will rise to the amount of the bracelets.
As long as you keep the receipts, calculate the Yen to dollar, and declare upon arrival in the US, you should be fine.
It’s the idiots who buy $100K worth of merchandise, take the detaxe, enter the US, and don’t declare — get caught, get fined, and or get the stuff confiscated.
If the pens you buy are made of rare and/or endangered CITES items, MOP, coral, and the like, I would check any potential customs issues prior to purchase.
ETA: I did not question the customs officials. I just answered their question truthfully.