The Gearbox Software Store is no longer selling add-on packs for Borderlands PC.
If you purchased your copy of Borderlands from a digital delivery service such as Steam or D2D, you will need to download the DLC for your copy through the service from which you purchased the game.
If you own the DVD version of Borderlands for PC, and previously purchased the DLC from the Gearbox Store, you can re-dowload your DLC here: https://store.gearboxsoftware.com/support/