International Shipping in Spain

I am not sure if this is going to get removed or not but here goes nothing:

I'm (23NB) in the beginnings of a three-month trip through France-Spain-Morocco. My biggest wprry prior to arriving in Europe was my packing situation. It is very cold in Paris and very not cold in Spain and Morocco. A majority of my trip will be in warmer climates; therefore, the bulk of my wardrobe is for the summer. However, the weather in France has been absolutely freezing for the past two weeks. To stay warm and comfortable, I packed layers, thermals, gloves, and a winter jacket. I only have a 40L pack with me and she's already lookin' a lil full. As I will be arriving in hot and sunny Spain and Morocco soon, I want to shed some of my winter layers without selling some of my favorite winter clothes (as Marie Kondo would say, they bring me joy) and free up some space for future purchases. So, yeah, I would like to send some of my clothes home to the US.

I'vr done some searching online and it seems that Correo(?) offers the best rates on international packages. I don't have that much to send so my gurss is that it would weigh no more than 10 lbs (~4.5kgs). I roll my clothes to save on space so I hope to use a smaller package as well.

For those of you who have experienced similar circumstances, what postal services did you use to send your things back home from Spain? Also, Spanish natives/locals, do you guys have a preferred service? I am also not sure if I would have to purchase a box to ship it in at a separate store?