然後如果您的航班出現任何問題，航空公司會告訴您去購票 agent 求幫助 節省下來的 $ 不值得未來可能出現的麻煩

If you even can save a few bucks.  I hardly doubt there would be "agency" tickets for US-EU routes that are cheaper than on airlines sites.

US-China MIGHT be.