In the Role settings for the User Interface, you should disable the show Add button for some or all Roles. Users can still Add records, but not with the add button. This is the normal setup when you only want records created if they will be child record to a parent. (no orphans allowed).
As for pushing a button to create an invoice, this can be doe either with an Automation or a URL formula button or simply the native Add Child button that comes with every relationship. That will pop up the Add Record form and populate the [Related Parent], but leave the user to fill in the fields and save the child (invoice).
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Mark Shnier (YQC)
Quick Base Solution Provider
Your Quick Base Coach
http://QuickBaseCoach.commarkshnier2@gmail.com
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