Users who purchased the credits go from unknown prospects to customers. All transaction fees are consolidated for better efficiency. From that point on, you are

Users who purchased the credits go from unknown prospects to customers. All transaction fees are consolidated for better efficiency. From that point on, you are servicing a known paying customer rather than spending effort selling your wares. Each transaction that starts from a known person rather than an unknown user means the friction of that transaction can be much lower. The user can simply click "read this article (1 credit)" while logged in. Then the article becomes accessible to them forever. They can read it as long as they are logged in. As time goes on, you can increase the cost of credits to meet your needs, but outstanding credits are always honored, and purchased articles are always accessible in the user's "library".