I've seen this pattern across a dozen SaaS startups. The core stack is pretty standard: WhatsApp API (Cloud API or WABA), templated messages for billing, reminders, birthday nudges, and a webhook that talks back to your gym software.
The biggest mistake people make here is using unofficial sender tools. They feel like a quick win, but eventually you get banned and lose the channel entirely. It's like trying to scale a house on a shaky foundation - works until it doesn't.
For setup, you can go direct with Cloud API for full control, or use a provider to handle the plumbing. Something that lets you connect your system, automate template approvals, and manage replies without building everything from scratch. The key is getting opt-in before you send anything promotional - even a birthday offer. That's not just compliance, it's respecting the client relationship.
Most gyms I've worked with lean on this for payment reminders and renewal nudges, then layer in the softer stuff like invoices and special offers once the automation is tight. Build the engine first, then add the personalisation.
