I just got an email this morning advertising it. Some of the t-shirts are £2 or £3, but the vast majority are just £1. The majority are plain, but some have some nice designs and all in a variety of colours.
Standard delivery is only £3.99 and last time I used it it came within 24 hours. As this is listed under "Offer Sunday", I presume this is to expire at the end of the day. There's nothing to say this is available offline.