All the listings I've looked at that you  have as Free Postage also have some 'Additional Payment Instructions' at the bottom of the Description.

Sellers cannot add extra costs after the item is sold. If you offer free postage ( and I'm reading it on my computer not 'phone or ipad) which is also in your listing under Postage costs - that is what should be supplied.

It may be that all the 'Additional Payments' are already in the item cost - if so I apologise - but perhaps it needs to be made a little clearer that Free Postage is actually Free.