Your question is Incrementality Test Without User Randomization. Take a moment with it on the right.
Talk me through your thinking if you like. When you're confident, submit your answer and I'll grade it like a real screen (7/10 or better passes).
You work on a retail media team measuring whether a campaign drove incremental sales, but you cannot cleanly randomize exposure at the user level because delivery and matching are imperfect. You still need a credible experiment design that can estimate lift while accounting for spillovers and operational bias.
How would you think about incrementality testing for this campaign when user-level randomization is not feasible? Walk through the experiment design you would choose, how you would measure impact, and what risks could bias the estimate.