GOG, you need to implement a flag that that tells the user their purchase cannot be completed when it is due to the purchase order including DLC for an application/game that is not within the purchase order or is not already within the user's Library. A user can inadvertently add DLC to their wishlist not realizing that they did not add the main application/game to their wishlist as well or does not already have the application/game within their Library. This is especially frustrating for the user if their purchase order includes 50-100 games that were added from their wishlist during a Holiday Sale, such as the Black Friday sale, where the user must individually remove each item from their purchase order to capture which item is driving the inability to complete the purchase, facilitated by the 'PAY FOR YOUR ORDER NOW' button being inoperable.

Yes yes, the shop is lacking in usability. We complain year after year but they have problems fixing it.