That's fantastic, thanks!
On a tangentially related note, I now have a calculated field that adds up all of the values in the 'Amount' column. It works great, but i'd like to set it up so the Submit button is hidden unless the value in the calculated field is the same as the value in the 'Total Number of Items received' box.

I thought I could do this using conditional formatting, but the calculated field doesn't appear to exist in my list of fields...