He thinks you are a cop and he wants you to prove that you aren't, so he offers you to do something for him. You will get two options, choose one (deny the first to get the second).
He offers you to take a hit (smoke) with him to prove you are not a cop. Accept and James will give you a pipe. Afterwards, talk to James and the cleaning products will be delivered to Stan's. But wait...
James wants you to prove yourself by taking a bag of drugs to a place that's said to be riddled with cops. He does not want to show his face in case there are and instead sends you.
If you accept you are take to a apartment complex. You get the description of the buyer, so start exploring! The complex has 2 floors with 6 doors and 1 emergency exit each.
When you think you have the right person, give them the 'stuff'. They will give you money and then cops will appear. They are easily outrun, head to the emergency exit. If you choose the wrong person as the buyer, they will either tell you that you got the wring place or cops will show up with no way to escape.
Done right, you should end up outside the SuperGas again. Talk to James and that is one item off the list!