It was a genuine question as i work in a post office. Did you put stamps on them or did the post office print a lablel? Were your international cards less than 5mm thick and less than 30g? If not youve probably not put enough stamps on, especially the USA and canada ones as they are more than cards for europe.
Also the return address, you say it was 5cm x 2.5cm? So almost half the size of the letter? 5x5 inch square is 13cm approx. I wouldnt call that a small label?
Before you put them back in the post box id have taken them to a post office to get them checked for correct postage. Mistakes do happen and things do get returned to sender but having so many returned suggests a problem and that big returns label could be it but without seeing it its impossible to tell.
Something is getting lost in translation here, probably my fault but I just measured the address labels by how many I could get from top to bottom of the envelope, one on top or above the other, it was six, then from one side to the other, it was two and a half, so I could get 6.5 labels high and 2.5 labels across.
So unless I’ve made another mistake I can get 14 labels on the reverse of the envelope, this hardly constitutes one being almost half the size of the envelope.
I’ve never uploaded a photo on this site, but I understand that I can do it via imgur, I’ll attempt this today, as it’s 02.20 now and I’m still unsteady after a few large vodkas.
I bought the stamps in a Post Office, where the guy measured and weighed the cards, especially the international ones, he said that they were all in need of £1.70 stamps as that was the price for mail 20g and under to Europe AND U.S. and Canada.
Nothing was mentioned about customs declaration, as they were regular Christmas cards only.