customs duty on magazines

I received, in the post, an annual magazine from an association I belong to, along with a bill for 6.24 Euros.  How is the sum calculated on a free magazine?

Could it be only the postage cost due to the weight? Have you had this same mag before and perhaps never paid postage? If so then seems things have changed perhaps with the post cost, or could it be the mag owner wants people to go online and download the mag instead of producing it and sending it out, either way you look at it perhaps your free mag wont be so free in the future.

The email receipt said  aduanera.

Isn't that Portuguese? Did your mag go their then get redirected to Spain.

Maybe the same in Spanish but according to Google translate it is Customs

Some magazines have a price printed on them even though they are free to members or subscribers to organisations..

Thanks, but no, there is not a price on the Regimental Journal.