Is this the relationship you are using?
Invoices::InvoiceDate = Schedule::EventDate (Your field names may vary of course)
That might work to "put your invoice on the schedule" without any need for the user to do so.
If this is not how you've set this up, please describe the current set up.
So you "put an invoice into the schedule" by creating a related record in ShowdayInvoiceLink?
That can be automated with a script trigger so that no user action is required.
If you need a visual indicator that such a record has been created, you can use this calculation in a calculation field:
Not IsEmpty (ShowdayInvoiceLink::InvoiceID )
You can format this field as a checkbox group in the inspector if you assign it a value list of just 1 as it's only value. Then the field will appear as a checkbox that is checked only if a related record in ShowdayInvoiceLink exists.
You could also use boolean data formatting in the Inspector to format this field to show any label of 7 letters or less, such as Sched and UnSched for True and False (1 and 0 ) values.
The formula works and I used the field as a checkbox set with a value of 1.