Regular first class then, tbh it doesn't really matter, just get certificate of postage and it'll be enough.
It's just by sending through Special Delivery it will get more attention from the company.
That's about what i'd do.
I had a similar situation with another retailer, i'd bought a DVD set (long one), and realised after about a 8 weeks some of the disks were faulty*, so they arranged for me to send it back.
I did, they sent out a replacement - after a week the replacement didn't arrive, so I contacted them again, and they sent out a second replacement.
The first replacement set then arrived about 2-3 hours before the second (one via rm, one via courier).
They arranged for a courier to pick up the spare set, and were quite surprised I think at my honesty
*It was something like 30 disks and 120 hours.
My personal view is, if a company makes an honest mistake, or tries to correct a problem that results in me potentially profiting, i'll do my best to sort it out (especially if the company has given me consistently good service).
If on the other hand it's a company that's screwed me around that sends me something twice it's a different matter....
