AWS Authentication Problem with POST/PUT but GET/DELETE work fine

I have a small set of API end points using AWS API Gateway & Lamda developed with the Serverless framework. All work fine from using the Serveless CLI tool Though Postman, I get this error with POST and PUT commands.

{
    "message": "The request signature we calculated does not match the signature you provided. Check 
    your AWS Secret Access Key and signing method. Consult the service documentation for details."
}

However, GET and DELETE commands work fine. I have the authentication set at Collection level so all 4 types of queries use the same authentication. It has to be that POST and PUT have a request body and the other two donโ€™t. The requests must be getting assembled differently.

I saw one POSTMAN bug reported about a year ago with a similar issue trying to upload an attachment. It doesnโ€™t look like that bug got fixed.

Anyone else able to connect to AWS API Gateway with API Key and Secret key?