I know how to add an invoice/quotation link to an email cover letter, @FatB0b-1960. I'm happy to share the entire process with you so you can complete your tasks as soon as possible.
In the invoice form, select the Save and share link button. This way, QuickBooks will generate a link for you to include in your email. Proceed with the following steps:
Click the Plus icon and then select Invoice. If you have previously created invoices, you can open them by going to the Sales menu.
Pick Save and share link in the drop-down or press Ctrl Alt L.