You can tell Ubuntu to either forget the updating completely, or just install the important security updates. Go to the Ubuntu software center, then to "Edit" -> "Software sources".
Tell Ubuntu to check only for important security updates (I definitely would do that!), and uncheck all the other boxes. Furthermore, select the "Never" option under "Automatically check for updates". You will then have to run the updates (if any) manually, for example at the end of the month if you see that you still have some transfer volume left.
That said, I assume that you have more than one computer in your café, and your main problem is not so much downloading the updates once, but the fact that each and every computer attempts to download all of the updates.
EDIT: Here, as pointed out by akin1012, there is an easier way of making a local update cache using the squid proxy server. However, personally I prefer apt-mirror, because it is easier to ensure that all of the clients get the updates only from the mirror, and if the mirror breaks, then the clients will just stop downloading updates instead of suddenly trying to get them from the Internet.
EDIT2: it is true that apt-mirror will download a large amount of data initially, so possibly that is not the best option. apt-cacher-ng has been suggested, but I have no experience with that.
You can do it with installing apt-mirror on one of the machines and pointing all of the other machines to this one. Naturally, you must carefully select what to mirror, otherwise it will try to mirror the whole Ubuntu repository, and not just the important updates (which is probably what you want).
You will find a detailed description of the process here, but beware: you only want to mirror these sources that you actually use!
In short, you need to do the following:
Edit the configuration file (include only what you need)
Start the mirroring -- it will automatically download all relevant files
Install a local web server on the same machine and point it to the downloaded mirrors
Configure all remaining machines to use that mirror instead of the regular sources.
This is all rather complex, but may be worth it.