I placed an order last night for a pair of Starfinder2 Pocket Edition books, and saw the option to add to my next subscription shipment. I decided I could wait, and hit that button. I then got a message that four items were successfully added to my next subscription order.
Four?
It seems that instead of adding just the pocket edition, which is what I had selected, it also added the hardcover edition, the special edition cover, and the retail cover edition to the upcoming shipment. I manually removed the books I didn't want, but that is something you might want to look into as I doubt few people want to order all four versions of the books and might catch a few off guard if they aren't paying attention.
BTW, this happened both when I tried to add the SF2 Galaxy Guide and the Player Core.
To be clear, I'm good. You don't need to respond to me about this unless you have questions. But I wanted you to be aware of the problem so you can look into it for the future.