If you're sending out batches of free links, you can use our Newsletters and Updates service to automate that for a nominal fee:
http://www.e-junkie.com/ej/help.updates.htm
Even if you're not sending file Updates to previous buyers of any particular product(s), you can create a dummy product and send us a list of names/emails to import to the Buyer Group list for that product -- contact support to request an import template file with further instructions:
https://www.e-junkie.com/ej/contact.htm
If you're frequently having to re-activate or replace expired links for buyers, you may simply need to edit the affected product settings to allow more Attempts and/or Hours. We recommend our default settings allowing up to 5 Attempts within 120 Hours or, if you prefer to be more restrictive, allow no fewer than 3 Attempts within 24 Hours.