As the title notes, when I action the checkout, the button to pay for items is greyed out, unselectable.
I pay with PayPal.

I purchased a game yesterday without problems.

I cannot even "purchase" free games (which do not trigger the link to PayPal's site), because the button is not available. (I can see it.)

You might want to refresh your account. If that doesn`t help, then contact support.

Unsuccessful.

Thanks for replying, though, I was unaware of the refresh process until just now. :)

Did you tried to click on paypal again?
Sometimes when I try to purchase something with my card, I have to select it again.

I have played "whack-a-mole" with all the buttons (though I on;y make purchases online with PayPal, so I haven't used another system).

No luck.

It's not a big deal, I was only going to buy Jagged Alliance 2, and maybe Castles. I have plenty of other games to play. :)

I have determined what the problem is: it was not a bug, but a feature.

I was trying to "purchase" the free extensions to Neverwinter Nights, but I erroneously mistook these to be for the original game, when in fact they are specifically for the Enhanced Edition.

Evidently my attempted purchase was blocked because it would mean that I had a (non-GoG) copy of Neverwinter Nights: Enhanced Edition. So, no French Extras, Spanish Extras, or German Extras for me!

For those interested, translations of the original game text can be found on the Interwebs, e.g., here.*

So, mea culpa (for poor reading comprehension, not for being an evil —accidental— pirate).

Small print:
* Somewhere in the first fifty pages or so.

edit: syntax

[EoT]

Actually, that isn't it. It's just, that GOG doesn't allow anyone to purchase a DLC pack or associated extra components if you don't already own the base game, as logically you won't be able to play it. Not particularly because of piracy. This information however, is something that GOG should deign to make a little clearer on the checkout page.

If that's true then they should also refrain from gifting the bleeding things, since I received The Witcher 3: Wild Hunt in some long-ago give-away, but (since the base game is beyond the specifications of my PC) I don't have any way to play it. Or, it seems, any way to hide it in my game list now that I do "own" it. (Though it sells for AU$60, so I guess I can live with it stuck in my games list. :)