int num = 153;
string hex = num.ToString("X2");//99
//byte send = byte.Parse(hex);//0x63 wrong!
byte send = byte.Parse(hex , System.Globalization.NumberStyles.AllowHexSpecifier); //0x99,Better to use TryParse.

//May be you are wrong twice，so 153 => "99" => 0x63 => "63" => 0x3F