Just give me your address I'll be right over to collect (and deliver it back)...............and I won't charge a penny ;-)
OK... now to be serious............
When it is collected you definitely need to get proof from the 'courier' or whoever collects. A signature or something, usually they come with a returns document. And don't lose it.
It is after all their mistake and not yours - so I wouldn't worry too much.
I have no legal qualifications. so all this advice is purely just my logic - and I don't know how trustworthy that is.
p.s if they have separate numbers for sales department and help or other departments, usually I find they pick up the phone much faster in the sales department.