Let me start by stating that I think RapidStart is basically a wonderful tool.
In order to use it most efficiently, I usually enter the first Journal Entry line manually, export the RapidStart package, and use that first journal entry as an example when creating the rest of the entries for any given spreadsheet within the RapidStart package.
In regard to inventory items, I created an inventory item manually in Financials with a "base unit of measure" of "PCS". Of course, "PCS" is one of defaults that are included in Financials when you first generate any new tenant.
Since I couldn't add item quantities through RapidStart, or "Export to Excel", I had to go item-by-item through the Item Card screen. When I tried to use the "Adjust Inventory" action, I would receive an error message telling me that "PCS" was not a valid base unit of measure. To get around this, I would click on the elipsis next to "base unit of measure" on the Item card, select a different unit of measure, save it, then hit the elipsis again, and re-select "PCS".
"PCS" is pretty easy to spell, but I actually used Excel's "Fill Down" function to populate this column in each row of my RapidStart Inventory Items spreadsheet, so I know it wasn't mis-entered. I'm guessing that your RapidStart validation doesn't trim this field when it uploads it to Financials. Apparently your lookup routine with the Item card also doesn't trim this field during the lookup process.
In any event, when you consider that quantities can't be entered via RapidStart, quantities can't be updated via "Export to Excel", and quantities can't be adjusted through the Item card without first re-selecting the "base unit of measure", you've managed to create an environment of almost total frustration!
In the end, it was humanly possible to get inventory items (along with on-hand quantities) into Financials, but because of a series of design shortcomings and bugs within your solution, this took far, far, far longer than it ever should have.