I have built payment systems before, but I have never used PayPal.
Previously with Barclays and SagePay, you bring your data together (price, item lines etc), encrypt the data using their services, then post the data back to their payment processing service.
I want to do this with PayPal, however I cannot find anything that seems remotely flexible enough to do the above.
I don't wish to use 'Buy now', 'Add to cart', 'Subscribe' buttons, I need to have the flexibility to manage that within the existing system. Any clues as to what I need?