Run every client from one Artec organization: one project per client, one subscription covering all of them (within your plan's project count).
Each project can point at its own billing card, so each client's campaign spend hits their own payment method.
Agencies also get white-label options: a custom subdomain (youragency.artec.app), your logo in the corner, and your accent color: so client-facing views look like yours.
Combine that with per-project team scoping and shareable campaign reports for clean client reporting.