Invoices are not created in MyCorbu. Rather MyCorbu helps you record and track your invoices. Invoices are entered as a series of transactions, one per active Project-Phase. And they make use of the Fees feature to facilitate this.
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Take a look at the HELP article to get an overview of how this feature can work for you. 
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https://www.mycorbu.com/help/invoices

Tracking your Project Fees is now possible with MyCorbu. The easiest way to see how it works is to visit the 'Test Drive' account. The Fees feature is found by clicking the FEES button found near the top of the Bookkeeping page. 

While working on Invoicing for MyCorbu, I stumbled upon a really handy invoice numbering system. 

Here is a link to a description.