If you could switch to a system where you download off your account rather than from an emailed link you could say just that: download link expires on ____ (in 1 week) with ___ downloads remaining. The terms of purchasing a digital pattern would be made clear before purchase, and you can include a reminder for people to back up their digital downloads in case of computer malfunction.
As for the people asking for a new link, it would still happen, but at some point you would just need to take a stance of “I’m sorry, we cannot reset the download link after ___.” (A month or so.) If someone lost or accidentally destroyed their paper pattern no one should expect you or any company to replace it, it would simply need to be purchased again. The same should apply to digital purchases too.