Rollup Summary – Sum up Child Record
There is no SUM operator feature with Opero Documents app. In order to sum child records in Salesforce, you need to use Rollup summary field for Master-Detail relationship Objects. If you want to sum up child records related through Lookup relationship, then the easiest way would be to use Lookup Rollup Summary 3rd party App: https://github.com/afawcett/declarative-lookup-rollup-summaries.
FYI this app will not work if using Professional Edition. Another option for rollup summary app if using Professional Edition is “Rollup Helper” you can find on AppExchange.