eJunkie FTW for what you're looking to do.
It sounds like
eJunkie will be your best bet to start with. For just $5 a month (For up to 10 products.) you can have them serve your file to buyers. You can still use PayPal or any other merchant checkout.
eJunkie will keep the file on their sever with password protection. You can setup parameters like how long the password is good for and such. They will get the PWD at the time of purchase and eJunkie will also mail the PWD & DL link to them.
If they forget the password or lose the link they can retrieve it through eJunkie.
$5 to try it out would be money well spent for what you are looking to do.
Here's an excerpt from this
link on their site:
Digital storage & delivery for files & codes
Instant and secure product delivery:If you are selling digital items, buyer is redirected to the instant download of your product after a successful payment. Buyer is also sent the download link(s) in the "thank you" e-mail customizable by you. These are secure links which expire after the maximum number of download attempts and hours you allow for the product.
Store your files securely on our server, or store them remotely on your own server:Either way, our system automatically issues each buyer their own unique, secure and expirable download link to obtain an exact copy of your file after their payment is confirmed as complete.
Works with stored pin codes as well as generated pin/registration codes:If you are selling phone cards or anything else which required stored codes, you can simply enter the codes while adding or editing your product and we will pick the codes from that list and send to the buyer. If you are selling a software or anything else need the registration code to be generated based on user data, we can call a script on your server with the user data and use the code generated by it.
Click here for details.
You can choose to redirect buyer to your own site after sale:You can choose to redirect the buyer your site after sale. It can be a simple redirect or a complex one where you receive sale data too.
Click here for details.
Send out free expirable download links:You can also use E-junkie to send free copies of your product to reviewers, friends or maybe just to yourself to test things.
PDF Stamping for E-book security:Each buyer's copy of your PDF eBook file can be stamped with their name, email address and unique transaction ID, which discourages sharing and allows you to identify who shared your file if you do happen to find a copy out "in the wild" somewhere.
Multiple payment choices for buyers
E-junkie Cart allows buyers to choose among
PayPal Website Payments Standard,
PayPal Website Payments Pro, PayPal Payflow Pro
(coming soon!),
Google Checkout,
TrialPay, or
Authorize.Net for their checkout.
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As AffAholic said, you will NOT stop all piracy, but you can limit it. eJunkie is a reasonably priced solution.
Just realize and ACCEPT that your products will get stolen and distributed freely. Do what you can within reason to make it more difficult. (Most people are lazy.)
Then just keep doing business.