We had a thread on this already. The pens are made by Pilot, they also have highlighters that erase. Both were made for students....now quilters and sewers have discovered them. I use them for tracing my redwork.
Heat erases the lines, cold (in frig or freezer) will bring them back. If you use an iron on the piece, then wash it before drying in dryer, the marks will be gone for good.
You can purcahse these at Staples, Walgreens for much less than LQS. They come in red, blue, green, black, purple and orange. Watch for them each year during back to school time. I bought them at 50% off plus a $4.50 rebate at Walgreens during back to school....so they were about .75 each.