I've been trying to book LAX-ATH via Google Flights & ITA for a few days now. Outbound is a Delta codeshare, returning is ITA, a few months out. Clicking through from Google Flights to ITA, first issue I encountered is I could not enter passenger middle name and get it to advance to the payment screen. I had to remove the (optional field) middle name and it finally worked. Then on the payment page, I have tried multiple credit cards and multiple browsers--every time I click "purchase" it pauses for several seconds and I get a message at the top of the page saying "re-enter credit card details". There is no block or pending transaction on any of the cards I tried. Any ideas? Super frustrating.