There is absolutely no way to get that many bags of groceries in a Spyder, even top down. A Spyder is quite a bit smaller in the interior than a Speedster. Imagine with the top UP! Honestly, you can't get into a Spyder with the top up gracefully. Even if you're tiny and a gymnast. Which is why the Beck boys put a folding frame on their top. You need to release the windshield header and fold the top back just to get in or out.
Carey and I did drive to a beer store in my Spyder at Carlisle one year, top down of course. We returned with three cases of beer. A 12 pack can fit in front of the cockpit bulkhead on each side. One case fit between Carey's legs, and the other one in his lap. That was one hell of a feat!