What tables and relationships do you have setup for this?
A typical invoicing system for a relational database such as FileMaker looks like this:
Unit prices and item descriptions are usually stored in Products_Services.
A Unit price is copied from Products_Services to Line_Items when a ProductID (may have different name) is entered or selected in a lineItems record via either an auto-enter calculation or a looked up value setting.
The item description can be set up to copy the description in exactly the same way or you can just include the item description field from Products in the row of your portal to LIneItems.
I would guess that you have a field with looked up value setting for your item description that has an error in it that keeps it from matching to a related record in Products_Services and specifys "New Item" with the "if no exact match" option.