Payments Standard is not a supported option for PowerStore. Implementing it would be custom and require hand coding in areas to make it work. I would not suggest doing it this way as it seems to be more trouble than it is worth.
If you use the Express Checkout only option it should function very similar to Payments Standard, except the user will be returned to your site to finish the transaction and you will get immediate confirmation. The flow with Payments Standard by comparison is not as simple and you need to rely on an IPN page to listen for the details about the transaction from PayPal, then have that page update the db and send email receipts.
You can still use Express Checkout to have users pay with their credit card on PayPal's site, you just need to check that this is an option for you, here is some more info about this:
PayPal has an "Account Optional" setting in your PayPal account Profile, under Payment Receiving Preferences. Ensure that it is on.
However, note that the PayPal account optional is not available for all locations, some locations (or countries, including China) are required by PayPal to have a PayPal account before making a transaction. In this same regard, if the IP address of the buyer is determined by the system to be at a location where the PayPal account optional is not available, they will be required to create a PayPal account.
If you are still having trouble with the Express Checkout API interaction and are not seeing all of the details let us know and I will ensure that a ticket is opened up for you.
