Samples can be accepted using any of the major archives, zip, rar or 7zip, provided the password is given in the body of the email there is no problem.
However, if you have a sample, like in the case of the one in your other topic, that can be in the chest without having to archive it and when you send samples from the chest, avast takes care of protection (the attachment is encrypted by avast), so you have nothing else to do other than add the file to the chest and send from the chest.