Use a layout based on the Item_pay table. You can perform a find for a specific customer on this layout or sort your records by Customer (if you want a report for multiple customers).
Summary fields defined in Item_Pay can compute the total paid for each Product_FK value if you put them in a sub summary layout part "when sorted by" Product_FK. You can include the individual item_pay entries or just the subtotals depending on whether or not you include a body layout part.
See this tutorial for more details: Creating Filemaker Pro summary reports--Tutorial
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