A few years ago, I ordered wiper blades online from ECP. They turned out to be the wrong size but as I had a day off, I decided to take them back to the nearest ECP shop and swap them for the right ones. The manager wasn't happy, and said I had to return them online. I read out the Ts & Cs from the back of the invoice - "Alternatively, you may return it to your nearest store." It all got resolved, but in a very grumpy fashion (them, not me).
Since then, I've always phoned my order through. Bizarrely there seem to be fewer mistakes.