Digital Paybox (formerly Donate & Download) is a plugin that allows to distribute files through payments. People can decide how much to pay and what payment gateway to use for that (PayPal, Payza/Alertpay, Skrill/Moneybookers, Authorize.Net, InterKassa, EgoPay, Perfect Money, Credit/Debit Cards (via Stripe), Bitcoins (via BitPay)), but you always can set minimum limit of their payments or even set fixed amount for each file separately. Once payment completed, the plugin automatically sends temporary encrypted download link to payer’s e-mail address.
Demo: http://codecanyon.net/item/digital-paybox-pay-and-download/2637036