Self-serve plan-change checkout flow

On my team, customers who wanted to change plans had to email support, and support would change it manually. My intern project was a self-serve plan-change flow in the existing React app: pick a new plan, see a clear before/after of what they'd pay and what changed, confirm. I didn't touch actual billing logic — I called an existing internal pricing endpoint that returned the proration preview, and a separate confirm endpoint that did the real change. Most of my work was frontend state: the flow had a preview step and a confirm step, and I had to make sure the price they saw in the preview was the exact one applied at confirm, because prorations can shift if you sit on the screen past midnight. I handled that by passing the preview's quote ID into the confirm call and letting the backend reject a stale quote, so we never silently charged a different number than what was shown. I also spent real time on the failure states — declined card, expired quote — because a half-finished plan change is scary for a small business owner. It went out behind a feature flag to a small percentage of accounts near the end of my internship.