For Odoo 15 Community, there isn't a built-in OCR solution like the one available through Odoo Enterprise's IAP services. Most Community OCR integrations rely on third-party providers, which is why many charge per document.

If you prefer a fixed monthly subscription, consider looking for integrations with services such as Nanonets, Rossum, Veryfi, or Mindee, as some offer subscription-based pricing depending on your usage. You can also integrate open-source OCR engines like Tesseract, but you'll typically need a custom module and the extraction accuracy may not match commercial solutions.

The best choice depends on your invoice volume, required accuracy, and whether you're comfortable with a custom integration versus a ready-made Odoo module.