The Correos will affix their own form, including the destination address. These bags are not handled as roughly as parcels or letter, as they cannot fit automated machinery smaller than a conveyor belt with a laser scanner.
The zip tie is a good thing. I use them religiously, even for domestic travel. I buy mine, in the small 4 - 5 inch size, in bulk and in a spectrum of neon colors, at my local Home Depot (your local DIY store) check the electrical section...
In my direct experience, I watch the Correos van pull up a the small Correos at the Pilgrim Office every day, when I am there volunteering. This truck comes to pick up the bags and bicycles collected from the satellite office, to go into the main flow. I can tell you that each bag is carried by the handles.
When you check your bag, the Correos person will wrap the bag with several turns of yellow Correos tape. It looks like police crime scene tape, but labeled with the Correos colors and logo. This tape holds down any loose straps and secures the zippers.
As I mentioned, they attach their shipping label to the bag. They did this to my suitcase at the Correos in Ponferrada the day before I started walking the Invierno this past April.
For 'insurance" I make business card sized "Destination" address labels using my computer and printer. For any travel instance, I make several labels for each pre-reserved destination, including my return home. this takes one sheet of 10 perforated cards. The cards look like this:
Mr. Firstname Lastname
c/o IBIS Porto Centro
Rua da Alegria 29
4000-041 Porto, Portugal
Tel: +35.1.223.400.700
The above is an actual example from a previous trip. I place these destination tags in the luggage tag attached to my luggage or the laundry sack containing my rucksack. This way, if my bag is misdirected, it will eventually show up where I am NOT back at my home in the US.
If you are mailing to Ivar, the label would likely look like this:
c/o Oficina del Peregrinos
Rua das Carretas 33
Santiago de Compostela
15705, a’Coruna, Espana
Re:
email.address@url.com
Adding your e-mail to the bottom allows Ivar to let you know that your bag has arrived.
Hope this helps.