I do post overseas but NOT to Spain (and many other countries.)  I had a bad experience with Spain many years ago (maybe 15?)  - at that time buyers had to pay duty at their end. The amount of duty demanded by Spanish Customs was much higher than it should have been  according to the buyer so they refused to pay and the parcel was returned to me. I refunded them. According to the buyer there was at the time a long-standing problem with Customs - I do not wish to cast aspersions so have no idea to what the current situation is.

I sell mainly one-off items collectables etc which may not be widely available, which is why I sell overseas (to some countries.)  I would have thought with the type of items you seem to sell (clothing including some pre-worn, cosmetics)  - that something similar would be available in any country, without needing to look overseas. There are additional problems with selling overseas; because I get additional sales that way I am prepared to wear them; only you can decide if it is worth it to you.