A normal report with 12 formula fields for the summations is what we usually do.

I've accomplished this before through using formulas that took advantage of NumberVars.

In this case my Group #1 is Item Code, Group #2 is InvoiceDate grouped monthly, and Group #3 is CustomerNo. I could accomplish what I need by placing the following formula in the InvoiceDate group header (suppressed) and the CustomerNo header (just to show the data) doing something like:

whileprintingrecords;

numbervar Jan;

Jan:=if month({AR_InvoiceHistoryHeader.InvoiceDate})= 1 then Jan+(Sum ({AR_InvoiceHistoryDetail.QuantityShipped}, {AR_InvoiceHistoryHeader.InvoiceDate}, "monthly")) else Jan

I could repeat that same type of formula 12 times, once for each month which is what I have done in the past. Is that the best way to go about accomplishing what I need? It just seems a bit messy.

Formula field JanQuantity, passing through the line quantity when the invoice is the correct month.  Sum this formula in a group header / footer (adding in a null test somewhere if you need to force "0" when there are no results).  Create similar fields for all the months.

This method gives you line data, so you can sum this type of field at different group levels without it getting too complicated.  Sure, this is not elegant, but it is simple and works.

Ok I'll bite, what's wrong with the Customer Sales History by Item in Sales Order? It lets you select the year and it displays all the months just as you are doing.

You are right, I would have been better off just adjusting the format of that report to what I need.