I've double-checked everything in my PayPal, and associated credit card, everything looks fine, but GOG is still returning a "payment declined" message for the sale bundles. Has anyone else hit this, have any suggestions?

different browser, clear cache

if youve done many purchases recently (like i did this morning) double check for the 'suspicious activity alert' email and remove the hold they put on your card

does GOG give the declined or Paypal (when GOG send you to the paypal pages for login + confirmation)

I had the same error with my VISA purchase a mere 15 minutes ago. Tried to complete the transation twice, both failed, but somehow a third transation completed (without me initiating it). Had a talk with my bank and they did not receive any transaction errors.

No idea what happend, but my issue seems to have resolved itself (no money lost and the transaction is complete).
I hope it works out for you too!

Yes, it's declining my VISA as well; GOG won't even let me try PayPal anymore, I seem to have locked myself out. It seems this sale is not to be, for me. Thanks for the help all.

It does not. Paypal won't help a card that's rejected.

Same problem for me with Paypal. Sent them several mails.
They say that they randomly refuse suspicious transactions.
I don't know how Paypal works in other countries.
In Germany you can either transfer money to your account or Paypal
gets it after a purchase from your bank account.
Funny thing, after i started to transfer beforehand there were no longer
suspicious deals....even if i buy several items separately from GOG in a
very short time.