Three Envelope Mockup
The best Envelope Mockup allows you to showcase your front and back envelope designs in a single mockup. Simply use smart object option for the design insertion and use well-named layers for the rest of customizations.
Free

Check out this collection of PSD letter mockups complete with matching envelopes, perfect for showcasing your stationery branding. Just pop your graphics in using the smart layer! You will be able to change the background color as per your choice.