This rules out the alternative hypothesis that mail-order sales have also increased across other companies. It also strengthens the causal connection between unlimited free and increased mail-order sales by suggesting that without it, such sales may not increase.

This weakens the argument by suggesting that many customers didn’t know about the company’s unlimited free shipping. If they didn’t know about it, it’s unlikely that it influenced their mail-order purchases.

Irrelevant. The manager only addresses an increase in mail-order sales, not an increase in profits from mail-order sales. The fact that profits increased doesn’t help to establish that the policy change caused the increase in sales.

Irrelevant. Even if other companies have been offering unlimited free shipping much longer, we still need to determine whether unlimited free shipping caused the increase in mail-order sales at the manager’s company.

Irrelevant. Even if most companies don’t offer unlimited free shipping, we still need to determine whether unlimited free shipping caused the increase in mail-order sales at the manager’s company.