If you don't already know how to make a selvage postcard, you will in 1 minute. Apply your selvages to stiff interfacing like Pellon (the non-fusible kind). Trim to a rectangle a little larger than your postcard stock. Attach your selvage block to the postcard by sewing a nice zigzag stitch around the outer edge. See the bottom photo.Trim. All done. Aren't these pretty? They travel well in the mail, and require a first class stamp.
These were made by Elena McDowell, and she told me how to make them. If you have been following this blog, you know that she is the one who received that 5 pound bag of selvages from her friends at a quilt shop. That explains how she can make such a fancy color arrangement:-)