I know label woes have been discused ad nauseum here before.
A fresh thought.
Avery makes a label that is all 'one piece', 8.5x11" for N American distribution. I presume there is a metric sized variant. By all one piece I mean that there is no separate divisions between the sticky backed paper on the page.
I have thought ' Why not format the addresses in your word processor so they look good on the page, and then print (or photocopy) onto this label material'.
Then cut it into strips to suit the text you have printed onto it, peel off the backing release sheet, and paste the now sticky exposed 'label' onto the envelope or postcard they were meant for.