Here are some options:

By the project:

Give us an objective, we’ll size up the task and give you a project proposal summarizing the work to be done, timeframe and cost.  Once you agree, we’ll execute.

By the transaction:

From press releases to promotional videos we’ll deliver your one-off assignment when you need it.

Retained support:

It’s useful to have the assurance and steady output of a dedicated resource.  We’ll structure a retainer that guarantees communications support on a regular schedule.  We’ll also be on-call for extraordinary demands.