Today I had such a parcel to send so I logged into Swiss Post, selected web Stamp, and entered my data. I find this site the opposite of intuitive so by the time I had selected the correct parcel type, price and entered all the required data it took the best part of a half-hour.
Then I reached the payment stage and clicked on "pay by credit". I got the message, "now you have to log in". But I am logged in! I can see the icon in the top right corner that shows I am logged in. Anyway I click on the login link and, of course, I cannot log in again.
Ok, so instead of using credit I will pay by credit card, so I click on credit card. Again I got the message, "now you have to log in". But I am logged in!
So, stupidly I tried logging out with the idea to log in again. As soon as I logged out all the data I had painstakingly typed in disapeared.