I have some more stuff to add to my shopping experience. First, if you're interested: check out this link which shows the path to "my" supermarket, starting at the East gate of Toyonaka campus (the International House is the first building inside the campus).
Then, the opening hours: for administrative stuff (like the alien registration office) and banks, the opening hours are as inconvenient in Japan as they are in Germany. For supermarkets, and especially for convenience stores, however, the opening hours are pure paradise. For example, the supermarket I went to is open from 9am to 11pm - on EVERY single day, including Sunday! And convenience stores are regularly open 24/7. So forgetting to buy groceries before the weekend will not lead straight into starvation, as it would in Germany ;-)
Convenience stores are just like small supermarkets. They have a smaller selection than regular supermarkets, and are more expensive. But their selection of take-away food is nearly as good as in other supermarkets. So they are very similar to the stores at German gas stations. But you can do some additional stuff there, like pay your electricity and utility bills (so you don't need a bank account for this!), and you can also prepay for orders on Amazon.jp.
Regarding Amazon.jp: I have ordered a book which will hopefully help me reduce the time I spend in supermarkets - the Guide to Food Buying in Japan supposedly explains how to find your way around the supermarket, pronounce Japanese writing, what to expect on labels and packaging, and so on. I only ordered it yesterday, so it hasn't arrived yet. I'm curious whether the address I gave them was really sufficient...
