Your question is Incrementality Test Without Full 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're measuring a retail media campaign where some exposure is driven by platform delivery rules, so you cannot cleanly randomize every user. You still need a credible estimate of incremental sales from the campaign.
How would you design an incrementality test for a retail media campaign when you can't fully randomize users?