From what I understand Express Checkout should be a stand alone option that PayPal offers, however in order to implement Express Checkout you will need to obtain the API credentials for this including the API username, password and signature. I don't know for sure if you can get these API credentials for free from PayPal without signing up for a Payments Pro account. I have been told that you just need to have a business account to make use of Express Checkout. You should talk to PayPal about this specific part to get more clarification.
If you are using a local checkout like Express Checkout I would suggest making use of an SSL cert for the checkout pages. Generally speaking it makes for a more comfortable user experience when the communication is encrypted in a checkout.
PayPal standard is not supported in PowerStore 2 or 3. In PowerStore 3 there is an option for Express Checkout only but it requires the same API credentials as a Payments Pro setup would.
Thanks Eric. I met with my client last night, and at least for the moment they have decided they do not want to pay for Express (or pro) so they are going to start off with Paypal Standard. I know that powerstore 2 (or 3) does not support pp standard, but am I correct in my understanding that as long as I have API credentials for pp standard (which I have) the customer clicks the payment button, is taken to paypal to make the payment then is returned to the site...the main difference is inventory control? Everything else works the same with all the confirmations and order registration?