プラグインのソースを眺めてみた感じでは、API のエラーをそのまま出力している感じです。
Amazon API のドキュメントhttps://docs.aws.amazon.com/AWSECommerceService/latest/DG/TroubleshootingApplications.html にも
Check how quickly your application is submitting requests.
If your application is submitting requests faster than once per second per IP address, you may receive error messages from the Product Advertising API until you decrease the rate of your requests.
とありますので、Amazon API 側の制限と思われますので、プラグイン側では対処できないと思われます。
同じページに
Effective 23-Jan-2019, the request limit for each account is calculated based on revenue performance attributed to calls to the Product Advertising API (PA API) during the last 30 days.
Each account used for Product Advertising API is allowed an initial usage limit of 8640 requests per day (TPD) subject to a maximum of 1 request per second (TPS). Your account will receive an additional 1 TPD for every 5 cents or 1 TPS (up to a maximum of 10) for every $4320 of shipped item revenue generated via the use of Product Advertising API for shipments in the last 30 days.
と書かれていて、
a maximum of 1 request per second
とあるので、毎秒1リクエストをこえると制限されるのではないでしょうか?
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こちらの条件を満たす必要があります。