Monolith
also: monolithic, monoliths
An app built and deployed as one single unit, where all the features live in the same codebase and run together.
An architecture where the whole application (pages, business logic, data access) is one deployable unit, usually one codebase and one database. A monolith can be well organized inside (a "modular monolith"). It's the sensible default for most small and medium apps.
A single restaurant with one kitchen that makes everything. Simpler to run than a food court of separate stalls, as long as the kitchen is well organized.
"Monolith" is sometimes said like an insult. It isn't. For a solo builder or small team, one well-organized app is usually easier to build, debug and deploy than several services.
Your app is a monolith, which is fine at this size. I'd organize the billing code into its own module rather than split it into a separate service.
Keep this as a monolith. Organize the code into clear modules for bookings, payments and notifications, but no separate services.
Thinking a monolith is bad or old-fashioned. The real problem is messy, tangled code, not being one deployable unit.