For US companies use their (typically Dublin) European HQs.
The maximum fine wasn't even achieved by Facebook, after years and many blatant GDPR cases. Do you really think someone is getting a fine for not replying to a subject access request in due time? If so I have a very good bridge to sell you, and that bridge has more probability to exist than Amazon getting any kind of GDPR fine for not acknowledging a SAR.
We are moving into the opposite direction. Drink a verification can.
However, they have a very generous free trial for sales/recruitment. You could probably activate it and get real support.