You want to LookUp today's price and have it in an invoice.
If the price goes up tomorrow, you do not want it to change.
A Looked Up Value is explained in the links in this Google
Google "filemaker looked up value site:filemaker.com"
Henry, is that you? Formerly from a St. Louis Motor Company?
I'd look at pointing to a calculation field with a lookup field. Lookups remain constant unless "re-lookuped"