Better get used to that. Nearly every time you buy something in Portugal they'll ask you for your tax id so that they can print it on the receipt. This is becau

Better get used to that. Nearly every time you buy something in Portugal they'll ask you for your tax id so that they can print it on the receipt. This is because this is something you can use for some tax breaks later and also there is some lottery going on I think? Though obviously the original reason for it to exist is to combat tax fraud. This is also entirely optional for those kind of purchases, and meaningless if you don't have one (which you probably aren't going to if you are just a tourist passing through the country) so you should just not fill that field. I seriously doubt it is mandatory.