The sentence is fine with "will" for two reasons.
First, "if" here is used in the sense of "whether", and the rule about not using "will" doesn't apply to the meaning of "whether".
Second, even a conditional sentence with "if" can have "will":
Can you assure me that the CD will be in stock in the next month? I'll order from you if it will be in stock.
The rule about not using "will" with "if" only applies when the condition is a future event. Most of the time, the "will" verb describes a future event, as in, "We'll move the party inside if it rains." But in this case, the condition is not a future event. The condition is the present knowledge of a future state. The buyer is making the decision now based on present knowledge of what's going to happen. It means something more like,
I'll order from you if I know that it will be in stock in the future.
The present knowledge is the condition, not the arrival of the CD in the future.