How do you make code taken from scratchpad work more than once in postman?

Hi there

So I'm looking to get items back from the affiliate api (think its known as PA API), I can use the scratchpad under the affiliate part and it works ok. If I then take this code as cURL and import this into postman it only works once and then I get the following error returned:

{ "__type": "com.amazon.paapi5#InvalidSignatureException", "Errors": [ { "Code": "InvalidSignature", "Message": "The request has not been correctly signed. If you are using an AWS SDK, requests are signed for you automatically; otherwise, go to https://webservices.amazon.co.uk/paapi5/documentation/sending-request.html#signing." } ] }

What I'm actually looking to do long term is take the code then from postman to use it in a C# application. Which would look something like this:

var client = new HttpClient();
var request = new HttpRequestMessage(HttpMethod.Post, "https://webservices.amazon.co.uk/paapi5/searchitems");
request.Headers.Add("Host", "webservices.amazon.co.uk");
request.Headers.Add("Accept", "application/json, text/javascript");
request.Headers.Add("Accept-Language", "en-US");
request.Headers.Add("X-Amz-Date", "20240308T155220Z");
request.Headers.Add("X-Amz-Target", "com.amazon.paapi5.v1.ProductAdvertisingAPIv1.SearchItems");
//request.Headers.Add("Content-Encoding", "amz-1.0");
request.Headers.Add("Authorization", "AWS4-HMAC-SHA256 Credential=###/20240308/eu-west-1/ProductAdvertisingAPI/aws4_request SignedHeaders=content-encoding;host;x-amz-date;x-amz-target  Signature=873f938eec02d205297acc823b89fc1de334ee2fca3d3ba017b0c98394676781");
var content = new StringContent("{\n\"Keywords\": \"yoshi\",\n\"Resources\": [\n\"Images.Primary.Medium\",\n\"ItemInfo.Title\",\n\"Offers.Listings.Condition\",\n\"Offers.Listings.Price\"\n],\n\"ItemCount\": 1,\n\"PartnerTag\": \"###\",\n\"PartnerType\": \"Associates\",\n\"Marketplace\": \"www.amazon.co.uk\"\n}", null, "application/json; charset=UTF-8");
request.Content = content;
var response = await client.SendAsync(request);
response.EnsureSuccessStatusCode();
Console.WriteLine(await response.Content.ReadAsStringAsync());

return Json("none");

I seem to of hit a wall here with my understanding, guess I need to sign each request perhaps:

https://docs.aws.amazon.com/IAM/latest/UserGuide/reference_aws-signing.html

I guess this has been before and there is code samples in C# as this would seem a very common thing to want to do, I've tried looking over the documentation (spent hours on this) I can't seem to get it work or even work out if what I'm doing is correct. Anyone out there done this before or can provide any insight?