You can initiate a refund through the Support system. This is a pre-order so refunds are fairly automatic if you qualify. Return of funds depends on your bank’s refund policy, but usually 5-7 days.
Here is the direct refund link for D2R with the menu options chosen for you. Blizzard Support - Contact Support
Support > Contact Support > D2R > Categorize Issue > Payments > refund.
The only exception would be if you got the bundle with D3 and played/leveled D3 chars. Then that would be considered used and seeing as they can’t split the bundle they usually won’t refund.