You can send a demand letter for breach of contract in several ways.
Certified Mail. Certified mail requires a signature from the receiving party to prove they got the letter. This is highly recommended for serving demand letters.
Personal Service. You may have a process server personally hand the demand letter to the recipient. This may be necessary when a party avoids service or is challenging to find.
Regular U.S. Mail. There is no requirement to send the breach of contract demand letter a certain way, and you can certainly choose to send it via regular U.S. mail. However, this method provides no evidence that the breaching party ever received the letter. This could have important ramifications at a later date.