You're unable to automatically attach a copy of your warranty policy to all invoices and have them sent. You'll want to attach these forms manually to each transaction and send them. I'll guide you how.
On the Invoice screen, click Attachments.
Browse the file you want to attach, then click Open to add.
Put a check-mark in the box beside Attach to email.
Explore this 3rd party app. You may put your warranty policy as engagement letter and will be sent automatically with your estimate/quote/proposal. Once your customer approved it, the app will generate and release the invoice automatically. It supports QBO.