Password Protected Excel Download using EPPLUS

Just need to use the .Save overload with a password as the option:

package.Save("password");

Response To Comments

To apply a password if saving via a byte array it is very similar:

Byte[] bin = pck.GetAsByteArray("password");
System.IO.File.WriteAllBytes(fullFilePath, bin);

It's not documented, but you can do as following:

package.Encryption.Password = "your password here";

Then serve your package with Save() or GetAsByteArray() of your choice