Below is an example of an Amazon Simple Pay button. I've replaced the hidden input description field with a select. This gives you a multiple choice drop-down that you can use for things like selecting a shirt size. Here's the full finished code.
<div align="center"> <form action="https://authorize.payments.amazon.com/pba/paypipeline" method="post"> <input type="hidden" name="amount" value="USD 22.45" > <input type="hidden" name="processImmediate" value="1" > <input type="hidden" name="signatureMethod" value="HmacSHA256" > <input type="hidden" name="accessKey" value="[key]" > <input type="hidden" name="collectShippingAddress" value="1" > <select name="description" style="margin-bottom: 10px;"> <option value="Awk Tee">Select Size</option> <option value="Small Awk Tee">Small</option> <option value="Medium Awk Tee">Medium</option> <option value="Large Awk Tee">Large</option> <option value="X-Large Awk Tee">X-Large</option> <option value="XX-Large Awk Tee">XX-Large</option> </select> <br> <input type="hidden" name="amazonPaymentsAccountId" value="[id]" > <input type="hidden" name="cobrandingStyle" value="logo" > <input type="hidden" name="signatureVersion" value="2" > <input type="hidden" name="immediateReturn" value="1" > <input type="hidden" name="signature" value="[signature]" > <input type="image" src="http://g-ecx.images-amazon.com/images/G/01/asp/golden_large_paynow_withmsg_whitebg.gif" border="0"> </form> </div>
The original line looked, which I replaced, was the following.
<input type="hidden" name="description" value="T-Shirt" >
That line is replaced with this select.
<select name="description" style="margin-bottom: 10px;"> <option value="Awk Tee">Select Size</option> <option value="Small Awk Tee">Small</option> <option value="Medium Awk Tee">Medium</option> <option value="Large Awk Tee">Large</option> <option value="X-Large Awk Tee">X-Large</option> <option value="XX-Large Awk Tee">XX-Large</option> </select>
You'll want to generate the full button using the Amazon Payments system and then make that minor modification to get your description.