Manager: Our company's mail-order sales have recently increased

Good question, with a couple really important takeaways if you don't mind me breaking down your post.

1) "how can I say that the reason for decrease in mail order of other companies..."

Note that his isn't your job at all! Your job in Strengthen questions is to strengthen the exact conclusion being given, which here is "our change in policy probably caused the increase" - the problem is about this one company, not all the others.

2) "...is definitely because"

Also important - in Strengthen questions your job isn't to prove something ("definitely") but instead to find a reason that makes it more likely.

What (A) does here is take away the possibility that this particular company's online sales are up primarily because everyone's sales are up - that it isn't a consequence of anything the company itself did, just a consequence of the industry overall doing really well. But if (A) is true, then there's something that this particular company is doing that the others aren't, and one thing we do know that's a difference is that this company has free shipping and the others don't. This doesn't prove at all that the policy is the reason for the increase, but by taking away the "no, everyone's doing well" possibility it does make the conclusion more likely to be true.