Once you start out, if you are in Spain, many of us will mail any items identified as surplus to needs, or just too damn heavy, to Ivar in Santiago. He operates a very convenient “warehouse” where he holds all manner of things shipped or mailed to him to hold.
About the only thing I cannot recall seeing in six years, is a shrink-wrapped spouse... just sayin...
Alternatively, if you have a hotel reservation at Santiago (you should BTW), you could contact them to ask if they will accept and hold a parcel for your arrival. I have never been refused.
Finally, if the item is of low value, or a consumable item, consider donating it at the nearest Albergue.
Assess, adapt, overcome...
We ALL have packed and carried too much at some point in our collective Camino experiences. It is sort of the “original sin” of the Camino. I suspect it is anxiety about the unknown and inexperience that causes overpacking.
With experience, you will learn too. The forum can help you avoid some of these packing issues. But, in the end, most all of us end up lightening our load. There is no shame to self second-guessing...
I have personally dumped the entire contents of my rucksack on the post office floor in a Pamplona to hold a field-tear down and repack. On my first few Caminos, I mailed so many Cana Verde Medianas to Ivar that he jokingly gave me his Camino Idiot’s discount. We are talking like one box a week for a month. I guess I was a slow learner..,
Hope this helps.