I was approved for the Amazon Product API, and am doing a couple test calls in ScratchPad, and it continually comes back with the error:
Error! TooManyRequests The request was denied due to request throttling. Please verify the number of requests made per second to the Amazon Product Advertising API.
I've never once received a good response. I have chatted into their support for over a week, and no one there is any help, and just says 'it looks fine, I don't know why you're getting that response'
- I've only made about 10 calls total, via ScratchPad. Maybe only about 3/day.
- I have more than 3 qualifying sales in the last 30 days.

- Keys are more than 72 hours old. Even made a second pair of keys, waited 3 days, and they still don't work.
- Applied for API usage and keys about 2 weeks ago, so hasn't been disabled due to 30 days of no sales attributed to API.

The issue isn't fixed yet with the scratchpad. It is well known that there is no solution for that problem at the moment. There are 2 good manuals out there for other Amazon affiliate usages: Workaround 1 and Workaround 2.
I think you should switch like in the Workarounds described. The API can't handle that by now.
I know that isn't the answer you want to here, but it could help others facing the same issue. I will keep the answer updated, if a final solution is coming up.