when you add the Paypal donation button to your page, it will create a form, you can add a radio group to the form named "amount" and add radio items to it for each suggested amount:
<input type="radio" name="amount" value="10.00" id="amount_0 selected">
<input type="radio" name="amount" value="15.00" id="amount_1">
the selected amount will be passed to paypal.
Confirming the payment is a part of the process when checking out through paypal. the customer adds the donation, and is brought to the paypal site to checkout, they elect the checkout method and then conform or cancel the transaction.