Are the profits from a postal savings account permissible (halal), given that they are variable and not fixed each year, and does this differ from an Islamic bank which also gives variable annual profits?

The savings account and the profits associated with it are precisely usury, because the Post Office adds a known percentage to the capital, and places the depositors' money in interest-based banks to earn a percentage from it, then distributes a portion of it to the depositors. This falls under what Allah has threatened those who consume it with war. Allah Almighty says: "O you who have believed, fear Allah and give up what remains [due to you] of usury, if you should be believers. And if you do not, then be informed of a war [against you] from Allah and His Messenger. But if you repent, you may have your principal – [thus] you do no wrong, nor are you wronged."