- The price is right.
- A clear transparent cover to place on top would be a good additional option and presumably would increase revenue for you :). Maybe you could put the stickers/logos there, as already suggested.
- An option for plastic spacers/screw/nut, to allow filing down the plastic nut to fit into Arduino holes. (I noted from your photos that you hit this problem with metal ones)
- A final note is that I tried to do a checkout on Amazon.com & it said it wouldn't ship to Ireland, Germany or UK (3 different attempts with 3 different addresses I have stored). Don't worry too much as I was only trying to see what the shipping cost to Ireland is.
Best of luck with this, as there seems to be a lack of good value enclosures / bases for Arduino compared to Raspberry Pi.