ECart will allow you to do advanced payment processing from your web site, and is probably what I would suggest for you. Powerstore does not sound like the appropriate solution since it would be overkill for your simple update.
You may not even need eCart if you are using Paypal standard.
You may want to sign up for a $99 premier support ticket to work with us over the phone and screen sharing to help you get started and determine what you want and need to get the result you are looking for. I'd suggest doing that first.