A modified Standard License image can’t be used for sale?

You need to read the licensing terms.

If the asset is the primary value of the good, like on a poster, a mug, a t-shirt, that you will need an extended licence. If something else is the primary value, a standard licence is OK.

So for a bookmark, it would be difficult to argue, that something else is the primary value. So I would say that you need to acquire an extended licence for this. This is true, even for a giveaway item.

The book presents a distinct narrative, and I sincerely hope that the narrative is the primary value of the product, with the cover being merely a supporting element. As a result, this would be acceptable under a standard licence, unless the print run (digital and paper) exceeds 500k.

Look here for more information on licensing: https://community.adobe.com/t5/stock/links-for-licensing-terms/td-p/11366788
(Disclaimer: As always with licensing, this is my interpretation of the rules. I think they are correct and advice is based on reading and interpreting the licence terms and on fair use for both the buyer and the artist/stock company, but I cannot rule out that my interpretation is wrong. I'm not an Adobe employee).

That is my interpretation. You need an extended License if you use the asset in any product for resale.

https://stock.adobe.com/license-terms