Unless there's a trick I'm not aware of, you'd have to define 5 different feidls for item number and 5 different fields for item description. Instead, define a second table of related records for these line items and use a portal to display them on your invoice layout. This gives you any number of items you need and your look ups will be quiet straight forward to implement. Another Forum Member, Comment, put together a simple demo file of an invoicing system you can use to see how this works and then enhance into a fully funcitoning invoice system: http://fmforums.com/forum/showpost.php?post/309136/
I initially did this in Filemaker 3 and I was able to have 5 iterations for each in that version of the software. Since it doesn't seem to work in Filemaker 11 (I did this work ~15? years ago and haven't looked at it since then so my knowledge is quite rusty) I think I'll just tell my friend (the user) to enter the Item Description by hand (which seems to work). Fortunately, the calculations still work.
Thanks so much for your help!
I'd check that original version if I were you. I doubt that you had look ups that successfully copied the values into different repetitions of the same field back then either. At least I never got it to work for me back then--each repetition would have to serve as a separate key field matching to your related table and FileMaker never worked that way.
Even in Filemaker 3, using repeating fields for invoice line items was a less than optimum design approach.