How do I download my monthly invoice for icloud storage?

If its from one of the last 90 days (3 months), you can get it from Reportaproblem.apple.com as explained above.

For older receipts, you will need to go through your purchase history on a device or computer. click here ➜ View your purchase history for the App Store, iTunes Store, and other Apple media services - Apple Support

Additionally you can access https://reportaproblem.apple.com and find your iCloud receipt there and take a screen shot.

You should be getting an email every time Apple charges you for something.