Bizarrely can’t make my local manual tests work
You may find it useful to stick a Console.WriteLine() as an else to if (signature == calcSig) . Something like
Console.WriteLine($"Calculated Signature: {calcSig}\nSignature from webhook: {signature}");
Keep in mind that when you are sending test hooks using Postman, you need to change the signature header value if you make any changes to the actual payload. When testing I would run it through my app, let it fail, and then just copy/paste the calculated signature into the header 