Sprout Invoices has had private unguessable URLs in it’s paid version since day one but sometimes added security is necessary for some invoices and/or estimates, this add-on provides that.
Adds the option to force clients to login or enter a definable password before viewing their estimates and invoices, and is fully compatible with the Client Dashboard feature[ref]Using private URLs for the Client Dashboard will not automatically login a client to view any invoices and/or estimates.[/ref].
Per invoice and estimate options allows for a secret password to be entered or select the option to force a client to login with their username and password.

Default view for forcing a login[ref]CSS can be removed and custom CSS can be added by removing the si_head::maybe_add_login_scripts[/ref] .

Default password protection view.

Additional Features:
- A default is available to force logins on all estimates and invoices, option is found on the General Settings tab.
- A new shortcode for Notifications is available to provide the password, if one is set.