We've implemented a fabulous new large window envelope for US business letters, which will allow you to have your logo and address appear above the addressee. Great branding!
Remember that all printed mail needs images that are 300 dpi (dots per inch) or they'll appear fuzzy when printed. Your screen is fine with 72 or 96 dpi images, but printed materials need higher resolutions.
Creating your letter is the same as creating an email, with the same editor you're used to. Adding images, changing fonts, bold, highlighting, and personalization is all the same.
Here's what page 1 of your letter looks like. The pink ADDRESS BOX is where the recipient's name and address will appear.
Your logo should be 385 pixels wide by 125 pixels high, with the image bottom justified so it will appear in the window. Include your return address below.
Here's an example of our own business letter. Make sure you start your letter below the pink address block.
Here's how it looks in the preview:
Here's how this looks with our logo: