Why did you post a non-answer? 

For future reference because you might be an AI.

An invoice is not a check. 

A check is money going out.

An invoice is money coming in

A vendor cannot be paid with an invoice. 

Therefore, explaining invoices is a pointless task in the conversation. 

 What would be useful is finding and sharing the add-on or upgrade required.

Alternatively, recommending another program or service.