What tables and relationships do you have now?
If you have this standard set up for invoicing:
where you individual items purchased are entered in LineItems via a portal on Invoices,
Then you can set up a summary report on LineItems where you can specify a date range for invoice dates plus any other criteria needed.
The report can have one row for each type of item purchased with total qty and total sales dollars for each item type. The report can include data from all three of the other related tables as needed to fill in what you need for your report.
We don't have this at all, actually. I don't think there's a single one-to-many reationship in here.
You may want to examine this simple demo file to see how this set up works: http://fmforums.com/forum/showpost.php?post/309136/
The Invoices starter solutions in FileMaker 11 and 12 are also based on this same set of tables/relationships but as part of a more complex design, so this demo file is easier to analyze to figure out how it works.