I think you should be able to figure out a relevant gift or bonus. This wild flailing around, trying to bribe subscribers is more of a sign your newsletter isn't up to snuff.
People want marketing to make people believe things that aren't true about things that aren't there, take actions which contradict self-interest. That's mesmerism.
If a newsletter can't offer some relevant information product, maybe upgrade the newsletter. Pumping up vanity metrics like body count isn't promising. Giving away Amazon gift cards validates Amazon has a viable business. Everybody knew that.