Hello, I would like to know whether there is a logo or seal of Let’s Encrypt that I may use in my online shops to display the I encrypted my stores with Let’s Encrypt ?
Let's Encrypt doesn't have a site seal or anything like that for various reasons. Some details can be found here:
You are, of course, free to include something like a generic lock icon and some text mentioning that the connection is encrypted, if you feel that would be beneficial.