I'm Spanish and some times I by some vitamins in the USA , my experiences are are that if your package is at the customs Agencia Tributaria , sometimes take about 2 hours to get it ( if you go there personaly ) .
If you do it through Correos ( post office) it would take a few days and a lot of money ( a size of shoes box is about 60 Euros , very expensive for the value of the products .
To tell you the truth is like a nightmare !!
So my recommendation if you don't really need the product is tell them to send it back home .
If you go personally to The Charge Terminal ( Terminal de Cargas ) in Madrid Barajas Aeroport Adolfo Suarez It would cost you something like maybe 15 or 20 Euros ( but is very complicate to handel it ( first you go to Post office Correos , is not easy to get there , then you go Agencia Tributaria is about 10 minutes walking ,fill a full paper after you have waited a for a while here and in Correos , Then you go to pay to the bank in another building , go again to Agencia Tribitaria ( wait) , say you are no introducing medicine or any other product ( say better vitamins ).
If they don't believe you it could stay for weeks before it get cheket by experts , if they trust you it would be given to you .
As I say before , if it goes for good two hours more or less!!
I told you is a Hell ( I have been going thought that a couple of time ( that's way I don't order any more from outside the European Union!
Sorry to tell you all that !!
God lack and Buen Camino