Two things:
First, I don't see a GDPR problem with buy buttons. Typically, they don't generate any cookies other than those a person would get from dealing with that retailer in the first place. As long as you identify them in your cookie policy and have appropriate cookie consent in place, you aren't violating GDPR. GDPR doesn't say no cookies. It requires that people know what cookies are involved and have the option to say no to them. (By the way, with Amazon ones, anyway, they still function even without cookies being enabled. I've tried my own website incognito, not consented to cookies, and still been able to use things like those neat book previews.
Second, there are other ways to build mailing list subscriptions than by hitting your viewers in the face with popups. Although what Anarchist says echoes the research I've seen, I'm also tempted to conclude popups drive people away, too. I've never yet subscribed to a list from a popup. I do have a subscription form in the sidebar of my website, and I have started using BookFunnel as a way to fulfill free book offers and a way to participate in group giveaways. In the last four days (my first group giveaway on BF), I've had 205 downloads and picked up 87 new subscribers (a 42.4% conversion rate). Yes, I could have gotten more if I'd required subscriptions to download the books, but it's not clear whether doing it that way is consistent with the freely given consent required by GDPR.
Note that you need to have a good reader magnet to use that isn't in Select. Some people write a novella just for that purpose. Anyway, at least people who are drawn in that way are readers. (They may not always be buyers, but since the giveaway started, I am seeing some increase in sales.
There's nothing wrong with trying a popup, but be aware that there are other methods out there.