Jet2 website says they will be contacting customers in order of booking date to collect the payment. It also says if you cannot be contacted prior to departure, you will have to pay at check-in, i.e. they will not be taking the money unless they get in touch (see extract from website below).
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Please note that our system for collecting the additional tax involves contacting our customers in an order relating to their departure date. Therefore our call centre and sales desks are unable to assist with queries relating to this, in this instance. This will eliminate disruption at check-in therefore maintaining our smooth, stress free check-in process. In the event we cannot contact you prior to travel, you will need to pay this fee at check-in before you travel.