In OJS 2.4.x you can enable the issue purchase option under Journal Management > Fee Payment Options > Reader Fees. This will allow readers without a subscription to purchase just a single issue of interest.
Readers who have a valid subscription gain access to all issues that require a subscription.
With manual payment enabled, the principal contact should receive an email with the details of the item being paid for.
You will then need to manually mark the item as paid in OJS, e.g. marking an author submission fee as ‘Payment Received’, or in the case of subscriptions, create/renew a subscription for the user.
Although there’s a manual payment option for issue and article purchase, it’s not currently supported on the back-end so you won’t be able to actually assign issues or articles to users.