You really shouldn't use SQL Express for something like this unless you understand the limitations and how to deal with them. I use the express (free) versions all the time for testing, temp dbs, data migration, automating data import/export tasks from one format or another to one format or another, but I never use them to actually store any permanent or even semi-permanent/expirable data.
I'm going to agree with dba-one below as well. Shrinking is bad. Planning is good. And if I remember correctly, you can still keep your DB at 5GB as long as it has free space in it so you should definitely purge some data as opposed to shrinking.