Yeah, it'd have been awesome to be able to do that. By my count I have purchased 116 J-novel volumes from Amazon since October 2018. It wasn't until earlier this year I considered membership -- I'd have been aboard long ago if I'd known I was missing out on premiums with short stories, not just larger filesizes.
The membership never made sense to me, since I prefer to read fully edited books on my e-reader and dislike reading novels on my monitor or phone. This is my first premium credit sale, though I'd already snagged enough to add volumes 117-140 and another 15 preorders. I can't really justify rebuying 80+ volumes that contain stories I missed, though.
At the very least I'd love the new site to add an option to mark old volumes "purchased" once they get it working better.