Poste Restant means that Postage was already collected at the sending office. There should not be anything due at the delivery office.
Hey, FirBall, Why would you tell someone to contact International Inquiry Center at USPS for something mailed from the UK to Spain. No wonder you flunked the test for Postal janitor.
You need to stay on your Meds
bulkmailtech_usps and zafir are both correct.
Only the experts at the International Inquiry Center can answer your question.
Call the International Inquiry Center at 1-800-222-1811 to get started. The lummoxes at USPS will likely give you the runaround and you may have to call back several times before the jack legs take action.
You’ll need to have the following information about your package…
the item number that appears on the barcode
names of the sender and recipient
addresses of the sender and recipient
the sender’s telephone number
date of the mailing
Then, once the damage or loss is confirmed, an information packet and claim form will be mailed to you.
Also:
File a Mail Theft - Mail Not Received complaint at the postal inspection service web site.
http://ehome.uspis.gov/mailtheft/mlntrcv...

When I went to the central post office in Madrid to collect my package "poste restante" I was told I needed some kind of checking number which looked like a sticker on the package. Will this have been given to the people who sent the package in the UK? I really have no idea at what point this should have been obtained.