Hi Anne,
2Checkout already sends a e-mail to the customer whenever they complete an order. Although, If you would also like to send a custom e-mail automatically you will need to implement on your website. One possible solution would be to put an e-mailing script on the page the customers are returned to after a order.

- Chase

Unfortunately, 2Checkout do not provide functionality for e-mails sent out by 2Checkout.com to be edited. I’m sorry for this.

There are a multitude of different scripts that you could use to send an automatic e-mail after a purchase. All you will need to do is find one that works for you and then have it set up to execute when the customer is sent to your Approved or Pending URL. If you have a web developer for your site this would be a simple thing for them to do, although if you don’t have one you may want to either ask around the forum to other vendors who may have implemented this or look through your favorite search engine.

I was looking myself and I found this article below which has a very simple explaination of how to do this with PHP and an example script. I won’t be able to provide you assistance with editing this, although it should give you enough information to start in the right direction.
http://email.about.com/cs/phpemailtips/qt/et031202.htm

- Chase