"... when the Christmas rush is over."
The phrase is just giving an indication of time and is in the present of the time indicated.
To contrast this with "... if it will be in stock"
This conditional is a condition about a future condition, it is in the future of the time indicated by the main clause. It means (with possible explicit timeframes in brackets) "I will order it (tomorrow) if it will be in stock (next year)" The time of "being in stock" is in the future of the the time of ordering.
By contrast "Could you send it when the Christmas rush is over" the time of "the Christmas rush being over" is in the present of "sending".
Using future tense in an if or when clause is rare and specialised. Learners should use the present tense in these clauses unless they have a very specific and certain knowledge that the future tense is required. If you are not sure, it is probably wrong to use the future in a "when" clause.