For Tarsnap, I just use PayPal; I don't like it, but they're better than the alternatives.<p>Since you're in the UK, I'd recommend looking into Google Checkout -- I understand it's available in the US and UK, albeit not the rest of the world.  Then you can offer your customers a choice.<p>EDIT: I'm also looking forward to using Amazon Payments -- so far it's US-only, but I'm sure they'll be expanding to the rest of the world any decade now...