Postman and Amazon Product Advertising v5 api

I am attempting to use the Amazon Product Advertising API (version 5) with Postman. I have an account, yada-yada. I have AWS auth setup with keys and secrets, I am including the mandatory header fields and a valid body. Every single time I fire that request I get an InvalidSignatureException. I am wondering if Postman is signing the request properly.

Postman apparently signs the request using a “signature version 4” as is what the API uses, so it should work, right? All I can visibly see in the resulting Authorization (Postman versus API scratchpad) is that Postman adds an execute-api, whereas the API scratchpad adds a ProductAdvertisingAPI in the Credential part. Will this put a spanner in the works? Seems to!

Any help would be greatly appreciated.