Re: collect it?
UK mail order rights (e.g. returning non-faulty goods because you don't like them) cease to apply when you click and collect. If they mail it to your house, you have vastly more rights than if you take it from a shop, regardless of whether the payment was made online.
So my guess is they're avoiding the DSRs (Distance Selling Regulations) rather than the Posties.
*IANALBIPOOTI
Biting the hand that feeds IT
