Minimal Host Accounting software (this the right group?)

Perhaps I’m in the wrong group. I use vi and a terminal for web development, occasionally using netbeans for codes formatting, code re-factoring and dead code maintenance.

I need accounting software designed for small developers who maintain a dedicated server.

I have a small but growing number of customers, for whom I design and host the site. I charge a yearly fee instead of monthly, which they pay in advance. If they leave (no one ever has) my contract with them says I’ll pro-rate a refund.

Trouble is I have to remember when each bill comes due and it’s unprofessional of me to bill them late.

I can imagine writing a small mysql routine that runs from cron which sends them (and me) an email a week before each yearly bill is due. But there will be other tasks too, as the business grows.

Are there any small open source web host accounting packages I should know about? That keep billing records, including subsequent charges for site maintenance and expansion? So I an prepare statements and invoices?

How do others do this?