you won't be able to process payments online without an Internet merchant account, and these are extremely difficult to get. Banks (called 'acquirers' in the business) typically require 2 year's trading history, plus audited accounts, etc. It can be a long and daunting process, sometimes taking many months and even after waiting that long many companies have their applications turned down. Internet card transactions carry the highest risk rating of all.
As you rightly say, you'll need the services of a company like WorldPay - in fact they're the company I would recommend. They have an excellent reputation and what they don't know about secure online payment processing isn't worth knowing.
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