Amazon Payments dynamic button signature being ignored?

Question

I'm creating a dynamic button and generating a signature based on the doc and example code I found on Amazon (using C#). Everything works fine, the payments go through, etc, but when I try to break things by changing the price in the form, it still goes through. It seems like the signature is being ignored altogether.

My first question is, are there any reasons Amazon would ignore a signature?

Not that I think it'll help much, but here's the generated html.

<input type="image" src="https://authorize.payments-sandbox.amazon.com/pba/images/SMSubscribeWithOutLogo.png" border="0">
<input type="hidden" name="immediateReturn" value="0" >
<input type="hidden" name="signature" value="myGeneratedSig" >
<input type="hidden" name="cobrandingStyle" value="logo" >
<input type="hidden" name="accessKey" value="MyAccessKey" >
<input type="hidden" name="subscriptionPeriod" value="" >
<input type="hidden" name="description" value="My Test Subscription" >
<input type="hidden" name="signatureMethod" value="HmacSHA256" >
<input type="hidden" name="referenceId" value="MyReferenceID" >
<input type="hidden" name="returnUrl" value="mydomain.com/mytesturl.html" >
<input type="hidden" name="ipnUrl" value="mydomain.com/ipn.html" >
<input type="hidden" name="signatureVersion" value="2" >
<input type="hidden" name="collectShippingAddress" value="0" >
<input type="hidden" name="amount" value="USD 299.00" >
<input type="hidden" name="amazonPaymentsAccountId" value="MyPaymentsID" >
<input type="hidden" name="processImmediate" value="1" >
<input type="hidden" name="recurringFrequency" value="1 year" >
<input type="hidden" name="abandonUrl" value="mydomain.com/abandonurl.html" >

Answer 1

I figured it out. Turns out, I didn't have "Sign The Buttons" checked under "Manage Developer and Seller Preferences".