"Yourself" is usually a reflexive pronoun; it is an object, but it refers to the same thing as the subject. In a sentence like "You washed yourself", it is clear that "you" and "yourself" refer to the same person. Compare these two sentences:
He talked to him. [He spoke to another male person -- perfectly normal.]
He talked to himself. [He spoke, but not to anyone else -- perhaps he was going mad.]
Reflexive pronouns can also be used to emphasise the subject: "I made it myself" emphasises the point that I had no help from anyone else.
But in your sentence, "yourself" doesn't refer to the subject. Many people use reflexive pronouns as a sort of general-purpose emphasis, or because they think that reflexive pronouns are more formal than non-reflexive pronouns. This is non-standard English, and you should avoid this usage in formal letter-writing.
"Your good self" is grammatically correct, because it is not actually a reflexive pronoun. However, it may be seen as an attempt to flatter the recipient, and that's not acceptable in many cultures -- it comes across as insincere.
A simple, straightforward "you" is the best choice here. Formal letters should be polite, but brief and neutral in tone.