No need to get passport stamped
I petitioned to remove conditions in Feb. I also got a letter that said that my green card is extended 1 year. I travelled abroad in March to Mexico by road. And in August, I travelled to India by air. Both times, I travelled just with my plastic card and that letter. I never got my passport stamped. And both times, there was no problem.
In both cases I first showed them my plastic card only. And then the immigration officer specifically asked for the extension letter. And when i showed them that, I was permitted to enter.