The User is able to delete invoices without intervention from Purchase Plus.
Please follow the below steps:
1. From 'Dashboard', go to [Documents] [Invoices].
Type the Invoice number in the [Search for Invoices] search field and [Search].
2. Click the  button on the Invoice line, as below.
2. A pop-up window will appear, as below. In that window, click [Delete Invoice] button at the bottom right-hand side of the window.
Note: if the Invoice is Cancelled, you will need to reopen/reactivate the Invoice in order to delete the Invoice.
Please check out this article for more information, Process or Delete a Cancelled Invoice.