JavaScript Store Popup Development

I need a lightweight, clean-coded JavaScript popup that I can drop into my online store without touching the platform’s core files. The goal is to keep the solution flexible, so I can easily switch its purpose—collecting emails, announcing a new product, or pushing a limited-time discount—just by updating the copy or images in one place.

Here’s what matters most to me:
• No external dependencies beyond vanilla JS and a sprinkle of CSS; it has to load fast and play nicely with my existing theme.
• A simple settings block (JSON, data-attributes, or a tiny config file) where I can choose when it appears—on load, after X seconds, or on exit intent—and which pages it targets.
• Smooth visuals: fade/slide in, dimmed background overlay, close button, and a cookie or localStorage flag so visitors don’t see it again for a period I set.
• Clear instructions so I can swap graphics, text, and triggers myself without hunting through the code.

When it’s done, just hand me:
1. The compiled JS/CSS files.
2. A short README explaining installation, configuration, and how to adjust timing, targeting, and content.

If you’ve built similar popups before, especially with vanilla JavaScript or tiny libraries, let me see a quick demo or code snippet so I know we’re on the same page.