He guzzled some of the stolen whiskey that Scott planted in the Dingle's barn. He promised to take the blame for the robbery (which wasn't a robbery) so that Sam and Zac would not have to go to prison, especially Sam.
In the past few days, he has been making noises about maybe taking them down with him, so they promised him a night with a woman to get him to stick to the story.
They had planned on getting him so drunk that he wouldn't remember whether anything happened or not, but apparantly Shadrach likes to be sober for romance - lovely.
Steph wanted to store her dodgy mobiles in the Dingle barn, so they were able to pressurise her into showing up, though she wasn't aware of Shadrach's lustful plans.