What can you do...? Download the full update to your PC and then install the maps which you require into your GPS unit as you need them.
To do that effectively you'll either need to delete your primary mapping file or replace it with a custom made one.
Start by installing City Navigator to your PC only which is an option on the download. If you already have this then don't bother with this step.
Method 1 (effectively delete primary mapping) - Then, from Mapsourec, write a new one tile (NORDSEE 1, being a chunk of North Sea without any roads, is only 5MB) gmapprom.img file to the internal storage of your GPS, this will overwrite your pre-installed mapping and leave the vast majority of your memory free. I believe that the 2820 has a mass storage option so this is relatively easy to do when compared to the SP 2720. You can then highlight the map tiles you do want and send these to your GPS by using send to device from within Mapsource. Every time you do this it will overwrite your mapping.
I'm not sure if the 2820 can get by without the presence of a gmapprom.img primary mapping file, you might want to try deleting it all together though it might throw an error on boot when it can't find one. Replacing it with a smaller NORDSEE 1 tile is probably the best approach.
Method 2 (replace primary mapping) - A better and more permanent solution is to write the tiles you want to a USB stick form Mapsource using 'send to device' and rename the resulting file from gmapsupp.img to gmapprom.img. Then transfer that to your GPS overwriting the existing file of the same name.
The geographical regional options offered on the map update download didn't offer the right areas for me so I used the method in the paragraph above. Somewhat more complicated with my 2720 as there's no mass storage mode, I needed to get involved with command line stuff and some Garmin VoDo.
Be warned that mapping transfers between PC and your GPS can take hours. You obviously need to have the mapping in Mapsourec unlocked for your specific Garmin unit, just because it works in Mapsource doesn't mean the transferred maps will work on your GPS