I can't speak for WA but PowerStore works as advertised.
From the FAQ:
Yes. Some of PowerStore's features (including inventory control) require local checkout to function. Due to the fact that local checkout is utilized PowerStore has been configured to use PayPal Payments Pro or Authorize.NET. The usage of any other PayPal options may lead to the loss of features.
What type of checkout does PowerStore utilize?
PowerStore utilizes local checkout which allows the entire payment process for your customers to occur directly on your website rather than having the customer directed to the payment gateway to complete their payment (remote checkout). WebAssist feels that local checkout provides the most professional and seamless user experience for customers therefore have implemented this type of checkout within PowerStore.
Do you have a return policy?
Since PowerStore is a non-tangible irrevocable (non-activated) product we do not issue refunds once the order has been completed and the product has been delivered via your Download Center.
PayPal charges a monthly fee to get the added features you are looking for. You should have a look at WebAssist eCart 4.