"I'll order from you if It will be in stock"
Gotube gave me this example here to show me that "will" was possible with if sentences, he said that in this case the condition was not a really future event and the rule of not using "will" did not apply there.
He then gave me another example "we'll move the party inside if it rains" where you couldn't use "will" because here the "will" described a really future event.
But when you say "We'll move the party inside if it rains" it is also because there is a present evidence of future state. May be there are some dark clouds in the sky or may be you've heard the weather report so the condition is not a future event like in the first example.
I am completly lost I don't understand the difference gotube tried to explain me.