Email Scipt or Program to Auto Reply Autorespond with Customizable Messages

We have a business online that sells digital download products through a site like Ebay. The site has no way to automatically send the digital downloads, so we have been manually sending out orders in an email using a link to each digital download that the customer purchases from us. We use a third party service that hosts all of our digital downloads and gives us a link to each digital download purchased We just found out from another seller on the site, that we could use Windows Live Mail as an autoresponder to send out our links automatically through Windows Live Mail message rules immediately when a Paypal Payment comes in with the item number(s) of the digital download. However the Windows Live Mail program is very limited. When a customer purchases more than one item from us, Windows Live Mail will send out a message to our customer for each item they purchased. So if they purchase 50 digital download products from us they will receive 50 different email messages (each containing a different link to their digital download). We have over 3000 items for sale and have to set individual message rules for each item that contains a link to that digital download. We often have customers purchases a variety of different items from us each time they buy. We've been researching and trying to find a product/script/or program that would allow us to automatically reply to messages when they come in with one email that contains all of the digital download links that a person has purchased from us. So instead of sending out multiple emails each containing a link to a different product, we would like to find a program that could somehow (maybe through mail merge, or some kind of data extraction) place all of the links into one email message according to which download products the customer purchased. Our customers purchase a variety of products each time they buy from us, so the system would have to be able to recognize item numbers and then mail merge short messages ( the links) into a main message. To explain it somewhat simpler terms, we need a system similar to Windows Live Mail that processes our messages right away when they come in, but that allows multiple different messages that we set up to be included (compiled ) in one main message according to parameters within the customers email message receipt that we receive. Everything will need to be completely automated and able to reply to messages immediately when they come in. We are also looking for a way to be able to extract the email address of each customer from the message and send to that email address instead of the email address that the message comes in from. Right now we can only use Paypal messages to autoreply because the message comes in using the customer email address and the Windows Live Mail will only recognize and send to the incoming email address and not an email address from inside the body of the message. However, we do accept credit cards through the site we use and when we receive these messages, the site we sell from is the sender and they send us a form that includes the customers email address and what they purchased from us. So we would like a way to send the message to the person in the email and not to the site who sends original form message. We are not tied into to using Windows Live Mail. Although we would be happy to use it if you can create something that works with it. We can also use Outlook or some other kind of mail program like Mailchimp, Aweber, etc. It just all needs to be automatic and work right when the messages come in. We want to be able to deliver all of our downloads instantly. We would be open to using any Mail program or Autoresponder program that would do all of this for us. Or if it's something you can create to work with Windows Live Mail that's fine, too. Whatever we can find to make it work would be great for us.