Lea Redmond's World's Smallest Postal Service is by far my favorite miniature version of an existing product. Redmond is a clever transcription service that lets you select up to six sentences for a miniature letter that is actually sealed and mailed.
According to Redmond's site, she uses stationery that is 1" x 1.5" and a microscopic ink pen. She seals, addresses and stamps these miniature letters in their own tiny envelopes with a personalized wax seal. Each completed miniature envelope then is placed into a clear case--with a magnifying glass, naturally--and is mailed to your chosen recipient.
While Redmond typically works out of stores and shops, she also takes orders on her website, which you can view at the Read More URL.