I use PhraseExpress (free version available) for answer e-mails as well. Gmail has boilerplate responses, but you can script PhraseExpress to retrieve data locally. So, if a customer is asking for an order, I can pull up details in the e-mail by doing a query locally.
I'm using PhraseExpress (free) too. I have a few macros set up for commonly used phrases, but thanks to its prefix matching and popup of multiple results, I also use it as a quick lookup tool for various strings so that I don't have to remember them.
Invoices and late payment reminders I've automated with my invoicing system (Crunch)