This pack goes on sale on Friday and contains the first two DLC addons for the PC version of Borderlands. The steam price is £6.30 each, so would be £12.60 for these two. Also some people prefer having software on disc anyway. It does not include the third DLC addon.
Thehut seem to have the best price for this, at £9.73. Also you can get your normal thehut cashback and/or discount (e.g. if you are a customer of morethan, you can get 7.5% off through http://more4me.morethan.com/
I'm assuming this works for both the disc and steam versions of Borderlands, if you know differently please post below.