You can only have one recordsource for a form/report and all the controls are either bound to that recordset or are calculated. There are various ways to obtain data from multiple tables/queries. The easiest of course is to include all the source tables/queries in a single query that joins them and use that for the recordsource for the form/report. In your case, you seem to want an aggregate function so you could not include that query in the form's recordsource because it would make the form non-updateable.
Another option is DLookup(), which can get a value from a query or a table. Queries and tables are interchangeable for most uses.
You might also be able to put a calculated control in the footer of the subform to sum the items on the invoice.
