I recently bought holiday flights with Air China through TravelUp and had a good experience overall. Admittedly I only purchased flights rather than a package and I did not need to make any changes to the booking post-purchase so did not incur any additional charges, nor did I need to speak with their customer services team. This was the first time I had purchased flights through a 3rd party and it was an anxiety-inducing experience (they ask you to 're-confirm' your flights a couple of days beforehand but don't explain why this might be necessary). However they were far cheaper than buying directly with the airline and it ended up to be just as smooth as if we had purchased directly (check-in 36 hours before flights all went smoothly).