Calculate Modern Square Inch Analysis
First of all, you're not going to lose all of the demand. Cannibalization of marketing activities causes demand to spread out across catalogs and email campaigns.
Second, you're going to learn how online items sell. Sum up all online demand for 1 million mailed customers and for 250,000 in the holdout group. Gross up total demand in the 250,000 holdout group by a factor of four to equalize circulation levels.
Then subtract the difference between the mailed and holdout groups. Add this difference to your phone demand. Finally, calculate DMPC on this total.
This method for calculating DMPC yields the true impact of catalog and/or email marketing activities. Now you're accurately assessing the productivity of each item.
You'll obtain results that are contrary to what you believe you know. Some items drive demand to other items; some items cannibalize other items. Calculate DMPC via A/B split tests with large holdout groups, and learn how customers truly interact with your business.