I think you should offer a subscription. You could offer different levels i.e.

$50/yr --> 50 credits per month, get double daily rewards
$100/yr --> 120 credits per month, double daily rewards, 5% artifact and research
$200/yr --> 250 credits per month, double daily reward, 5% artifact and research, Patron energy relay (+5 energy charge rate)

There could be different levels of Patron energy relay for the different years.
Level 1 Patron relay -- +5
Level 2 Patron relay -- +4
Level 3 -- +3
Level 4 -- +2
Level 5 -- +1
Level 6, 7,8, etc -- +1

You could still offer credits for sale the way you do now, but this would give you a stable player base, and also keep the players coming back.

This thread has already been made. Check A premium account level.

and was met with a resounding thud