The ornamental design for an envelope with a window, as shown and described.
FIG. 1 is a top plan view of our new design for an envelope with window.
FIG. 2 is a bottom plan view of FIG. 1.
FIG. 3 is a right side elevation view of FIG. 1.
FIG. 4 is a left side elevation view of FIG. 1
FIG. 5 is a back side elevation view of FIG. 1; and,
FIG. 6 is a front side elevation view of FIG. 1.
The dash-dash broken line showing of fold line and dash dot dash line showing boundary line are included for the purpose of illustrating environmental subject and form no part of the claimed design.