One thing you need to be aware of if you're thinking about using direct debit is that customers can get their money refunded, no questions asked, by simply telling their bank that there has been an error.
Interesting. Personal experience from a business owner point of view or as user?
I have a friend who added GoCardless as a payment option on their website. A customer signed up, paid using GoCardless, used the site for six months, then got a refund. GoCardless took the money out of the next payment my friend was set to receive and told them that they'd have to take the customer to court if they wanted to get their money back.