How can I relate Invoices to Opportunities?  I tried a custom field called Lookup and put Invoice which works, but I want a many to many relationship.  So the same way for example when you go to Accounts, it has a related list "Opportunities" and it lists all the opportunities under them.  Not sure if this is possible, but from Invoice I don't need to see anything regarding Opportunities there, it's just the Invoice is me invoicing closed/won opportunities, so I want to be able to see all invoices that an Opportunity appears on.  Just has to be many to many as sometimes invoicing is done in partial, so was hoping the way Opportunity shows:

Competitors

Attachments

Products

Etc, that it would have one that is Invoices and then show all related.  Can do this in Salesforce just unsure how to in Zoho, thanks!