Your code can be found in your customer profile. However, if you have difficulties finding it, you can also contact us via our Galaxus Assistant. https://www.galaxus.ch/help
You do not receive anything by post. You have received the purchase confirmation email and on it is the code to be sent to the person concerned. I have printed the document.
No, unfortunately this is not possible. Credit can only be added to a Nintendo Account from Switzerland. To add credit for another country/region, create a new account and buy the corresponding prepaid card from the retailer you trust.