I know you want to optimize and target and show things until I suddenly want to buy them despite previously not. Please resist that urge, at least optionally via a setting.
Drops I have seen are useless noise (to me). I am subscribed to your emails to see new drops, not to waste time seeing repeats. The noisier you make the process the less happy I am; annoying me as I'm looking through your wares is not good business.
Add a Kickstarteresque "Remind Me" button if you wish to prod people. On the rare occasion I don't just buy things I want (for the price) when I see them that would be useful. Otherwise, my looking at the drop and not joining was a signal that I didn't want to do so.
Furthermore, it's frustrating that not all drops seem to appear in emails. I'm not going to visit your site regularly to check for new stuff when I'm already subscribed to your mailing list. I only happened to notice this occurred via the "Other Drops" section when I clicked through your emails.
I'm a paying customer who is literally asking you to see what else I can buy, and you somehow don't just show me. The "show all drops" and "hide items which are not shippable" threads are asking the same thing; you're a store, and we want to easily see a list of what we can buy. Please listen.