Here is the short version of my 'horror' story from 2012.
I mailed 3 boxes of gear and gifts and stuff to my cousin in Santander, 10 days before our departure from the U.S.
We were going to visit Santander before heading to Sarria, and return there after Santiago. No brainer, right.
Except, some fool in Customs at the Madrid airport decided that we were trying to circumvent the import laws, and sent a letter to my cousin demanding a detailed list of the shipment contents, a 27% duty assessment, and a 12% administration fee. Needless to say we judged that to be out-of-the-question. Fortunately, we had checked a duffle with our backpacks and a few essentials, but the bulk of our well planned, tried-and-true hiking gear was "stuck in customs", as they say.
Hooray for Decathalon, and the Columbia knock-offs they stock. We shrugged off our first Camino challenge, and went on to our great adventure.
The boxes were finally returned a full six weeks after our return home. Because the U.S. Customs can be just as mussed up as their Euro-brethren, one of the boxes made 2 round trips before I was able to convince someone in Miami to manually handle it.
Lesson learned; carry essentials, always.