Why is billing an engineering problem?

Billing. It’s the kind of thing no one wants to think about until it’s too late. One day, you’re running a smooth operation, pushing product updates, making sales, and then you realize your billing system is a mess. Your invoices don’t match up, customers are getting overcharged (or worse, undercharged), tax compliance is a nightmare, and your engineers? They hate their lives.

Billing systems are like an intricate spider web. At first glance, it seems manageable—just a few connections here and there. But the deeper you go, the more you realize that every strand is linked to something critical: finance, product, customer experience, support, legal, compliance, sales, and more. And if one thread snaps? The whole web starts to unravel.

If you’re currently maintaining a homegrown billing system (or considering building one), let me save you some sleepless nights. Here’s why that’s a terrible idea.

Three Ways to Approach Billing:

Companies usually take one of these three routes:

It’s never that simple.

The 10 Pains of Billing (From Annoying to Absolute Chaos)

1. Enabling Multiple Payment Gateways

Want to support Stripe, PayPal, ACH, wire transfers? Each one has different rules, APIs, and compliance requirements. Now multiply that by international variations. Good luck.

2. Expanding to New Geographies

New markets mean new tax laws, invoicing rules, currencies, and compliance requirements. VAT, GST, local levies—it’s a nightmare. And it keeps changing.

3. Experimenting with Pricing and Entitlements

Launching new pricing models? Want to test different entitlements for enterprise vs SMB? Without a robust system, every tweak requires weeks of engineering effort.

4. Handling Upgrades & Downgrades (Enterprise vs SMBs)

Enterprise customers negotiate special deals, while SMBs expect smooth self-serve upgrades/downgrades. Managing both without breaking everything is harder than you think.

5. Proration, Refunds & Credits

Should you prorate upgrades? What about downgrades? Store credits vs refunds? If you don’t get this right, expect angry customers and revenue leaks.

6. Usage-Based Billing (So Many Ways to Get It Wrong)

Real-time vs batch metering, handling overages, dealing with different billing cycles—it’s an infinite headache. And customers expect accuracy.

7. Subscription Lifecycle Management (Pauses, Cancellations, Churn)

Pausing, resuming, retrying payments, handling chargebacks—billing isn’t just about charging customers. It’s about keeping them paying.

8. Custom Contracts & One-Off Deals

Sales teams love making custom deals. Billing teams hate implementing them. Enterprise contracts don’t fit into standard billing logic, and retrofitting them is a mess.

9. Selective Pricing Changes & Legacy Plans

New pricing should only apply to new customers, right? But now you have multiple pricing versions coexisting forever. This technical debt will haunt you.

Why This Stuff Is Hard?

Some billing challenges pop up once and are solved forever. Others evolve constantly.

Billing isn’t just an engineering problem—it’s a finance, compliance, and customer experience problem wrapped in a thousand layers of complexity.

What Should You Do Instead?

I’ll say it plainly: The best billing system is the one you don’t build yourself. Billing is deceptively simple at first. But it’s an iceberg—the deeper you go, the worse it gets.

If you’re still thinking, “Nah, we’ll just build a simple system,” I have one word for you: don’t. Hence, we at Flexprice blatantly say: "Developers should focus on building, not billing."