Stockashop offers full integration with the SagePay payment processing. (SagePay used to be called Protx by the way.)
SagePay is a "secure online gateway for credit card processing", meaning that they deposit funds directly into your business bank account (unlike PayPal which uses a separate PayPal account). Their small business service costs £20 per month with no setup fees and covers up to 1000 transactions per quarter (10p per transaction above that). Note that you will also need an internet merchant account, which will also have associated fees.
To use SagePay with your Stockashop, you will need to:
Your customers will be able to pay by credit or debit card.