Here is an SVG file for a 3×5″ envelope with a shaped flap. Set the height at 6.5 inches to cut a 3×5″ envelope. The layered file has a layer for scoring with solid lines. The SCAL1 file does not have score lines.
The download includes a layered svg for use with SCAL 2.0, a single layer svg for use with SCAL 1.0, and the png file for printing and hand cutting. These files are for personal use only. Feel free to link to this post, but the files are not to be shared or distributed in any way.
Download the files from here.