Half a million records for a six-month period suggests that you are bringing in less than 3000 submissions a day, on average. This is not a particularly large number from a flat file perspective -- I wouldn't worry too much about optimization unless you have a practice of really going out of your way to make your code inefficient. :)

I know you've said you don't have access to a database, but sometimes people say that when they mean "I don't have access to an instance of Oracle" or whatever the company uses for production data. Have you considered MySQL? It is pretty easy to install, and has most of the power of a 'full-fledged' commercial relational database. Encryption/decryption aside (because I know very little about such things), having your data stored in a relational database makes the extraction of that data in interesting ways much easier.

Rather than blindly hand over the CSV file or whatever to your customers, I would go a step further and ask, "What do you do with this information?" I find that many times there is some simple task I can do while I still have the data that makes life much easier for my customers.