Hello there, @Jose. I'm here to help you add an attachment to your sales forms when sending them to your customer in QuickBooks Online (QBO). 

Before you send an invoice or estimate, you'll have to ensure that the Attach to email option is checked so your customer will see it. Please refer to the screenshot below for visual reference:

For more information about adding attachments in QBO, check out this article: Add or delete attachments in QuickBooks Online.

You can also customise your sales forms to include only the details you need. For the detailed steps, read this link: Customise invoices, estimates, and sales receipts in QuickBooks Online.

I'm always here ready to lend a hand if you have any other concerns managing sales transactions. Have a great day ahead and stay safe.