Sure, E-junkie can handle the variable payments. All you need to do it to is to set the product price to 0.00 while adding your product in E-junkie admin and the ADD TO CART button you get will let your users enter their own amount and checkout using the payment methods you have seleceted (paypal standard, paypal pro, google checkout, authorize.net)
If you want to update the user accounts automatically after the payment, take a look at the "integration" section at http://www.e-junkie.com/ej/help.php
