Amazon Simple Email Service (SES) makes it easy, and cheap, to send emails reliably from a website. Here’s how to set it up for use with multiple domains.
When you configure exim4 as your email transport, you will likely need to tell it to replace some user names with aliases. To make your life simpler, use wildcards to do the job for you.
The wp-e-commerce shopping cart plugin is probably the most popular cart plugin for WordPress, but it isn’t always the most easily customised; in fact, theming it can be a nightmare. One little niggle I had to fix recently was that the notification emails don’t include relevant purchase information such as which shipping method was selected, or the per-item comments from the purchaser (e.g. a message to be included inside a book flap).