The process that you have just described is for Express Checkout. If you have the api credentials you should be able to make use of this in either PowerStore 2 or 3. If you want to make it an Express Checkout only scenario you would just need to comment out the form on the checkout page.
Payments Standard works with your PayPal id and not an API set of credentials. With Payments Standard you will not stay on the site, you will be taken to PayPal to complete the transaction, then you will be sent to the page that PayPal returns you to. You will not know the status of the transaction at this time. You would need to rely on an IPN page to handle the information about the status of the transaction.