there may be some general purpose invoicing software you could use. There are some online ones, like Freshbooks (I have used this, and while it doesn't have tons of features, it's fairly solid).
Building your own is indeed more work, but it sounds like you don't really have a whole lot to track. If you are using MySQL and PHP/Ruby/etc, you could probably find a small VPS and host on that, get a cheap SSL cert (you can get "QuickSSL" or "RapidSSL" style certs for $10-20) and be on your way.
So what tables would you need? Well, to get started, probably something like:
- Customers (basic demographics - name/address/phone and maybe a few custom fields as needed)
- Products (or some variation thereof, so you can track what you are selling, and what the prices are - but may not be needed at all if you manually create each invoice)
- Invoices
- InvoiceItems (optional to hold the line items on the invoice)
- Payments
Depending on how adventurous you are, you could even develop the website using HTML5 and create an "offline" version using some of the new features in HTML to let you run the app, locally, on your phone when not connected to the internet.
Here are some links to get you started with a few of those features
http://diveintohtml5.info/storage.html
http://en.wikipedia.org/wiki/Cache_manifest_in_HTML5