Buy one get one free on Oxford World Classics @ Borders

Starting from £3.99.
Wordsworth Classics would probably be cheaper, but a good deal on Oxford Classics nonetheless.

Free delivery on spends over £15, but I think you can get it delivered to your nearest store for free. The 20% off voucher doesn't seem to work. I don't see why the £5 off £30 spend voucher shouldn't work, however I'm not 100% on this matter.