I think there is some confusion about the payment options that are offered by PayPal and supported by PowerStore. The type of checkout you are referring to is a Payments Pro checkout.
A Payments Pro setup gives you two choices for paying, you can either pay with a credit card directly on the site, or login to PayPal and finish the transaction. The method of paying by credit card is called Direct Pay. This is only available to you if you have Payments Pro on your PayPal account. The other method of logging into PayPal then returning to the site to complete the transaction is called Express Checkout. You can have only the Express Checkout implemented if you choose to. There is no monthly fee for this service. This is the type of checkout that you will be using if you choose the 'Express Checkout Only' option for the gateway.
This is why I was stating that the Payments Pro is not the only PayPal checkout option. I found this great page on PayPal's site that gives you some of the details:
In the pricing tab you can see that there is no monthly fee or setup costs for this service.