When a customer run's out of credits in their package, this is an opportune time to prompt them to purchase another pack. This prompt can be automated using Sequences.
Sequences allow you to tell Momence "when x happens (we call this a Trigger), do y (an Action). In this case, the Y can be a message to a customer that includes a Purchase link to buy another package.
To create a Sequence,
click Marketing > Campaigns > Create sequence > define the event that causes your message to fire > create the message > enable the sequence