What you describe is typical of any invoice or purchase order system. In fileMaker it's built arround 3 tables with relationships. The only difference in terms of the database between a purchase order and an invoice being the name of one table:
PurchaseOrder----<LineItems>------Products (---< means on to many)
On your purchaseOrder layout to put a portal to lineItems with a scroll bar included so that you can enter a list of as many items purchased as needed for that purchase order. A different layout is use to print them out that properly adjusts for the varying number of items.
Both the purchase order and invoices starter solutions that come with FileMaker have this basic structure but with a number of added features you may or may not need.
EDIT: Note the following was posted prior to reading PhilModJunk's response and is not a follow up to his post but to the original.
As a follow up, it would be handy to have a field for the number of items purchased on a given receipt. When that number is entered, the record dynamically generates the needed number of Item fields and, then, when data is entered in that Item field, which would preferably be an editable drop-down menu populated by items in a value list, the sub fields needed for that item are revealed.
That is, on the new record for the receipt with 3 things purchased, I enter 3 in the "number of items purchased" field. That adds to the record three Item fields. When I select Blueberry muffins from the drop down menu for Item 1, the sub fields qty, costs, number and type of item is revealed.