The mail order pharmacy capability may have occurred under his short tenure as president.
It wasn't, however, what Carter would call his signature piece of legislation.
In fact, many historians would argue that Carter had no such legislation.
Some would argue that Carter was noted for Camp David Accords and The Canal Zone Treaties and give back of the Panama Canal.
http://www.jimmycarterlibrary.gov/documents/keylegis.phtml
As far as Carter fiddling with our nation's health care, his key legislation was Hospital Cost Containment Act of 1977 (H.R. 6575 - S.1391).
It was never passed by Congress, only the Senate, thus never signed into law.
Carter was able to effect some voluntary health care cost reduction measures during his time in the White House.
http://content.healthaffairs.org/content/9/3/66.full.pdf
Mail order pharmacies have operated for a century in the US.
The major focus was to serve rural areas.
The US Veterans Administration began using mail order refills in the late 1940s, early 1950s, and the growth continued.
The OP makes a great point, in that mail order pharmacies can save patients money, planning must be paramount.
Medical problems can get in the way of such planning, as can items lost in the mail, misdirected, or delayed.
One could argue that the current occupant of the White House (whose lease will soon expire), had little hand directly in the increase of mail order pharmacies use by certain health insurers.
One could also argue, that is something isn't broken, its folly to call fiddling wit it, FIXING it.
My wife must use certain medications regularly.
She chooses to play the Tri-Care RX game.
I, on the other hand, bought what some would call a "platinum plan".
I pay dearly for it.
I'm grateful I can afford it.
My wife didn't want me to add her to mine, as I use Tri-Care as a tertiary insurer.
She manages the mail order minefields, and has received her meds on time.
Me? I obtain 30 day supplies, as needed, from my local brick and mortar pharmacies.
With each visit, I'm hounded to "join" the 90 day mail order scheme.
I smile, and decline.
I fear the day will come, if I live long enough, that my ability to choose will be taken from me by those who profess to know better what works for me.
Which is their way of beguiling most people into being used to pay for the needs of others.