I have seen Ivar's "warehouse" many times... Based on my direct observations, about the only thing the Correos will not ship is a live pilgrim.
I have seen all manner and sizes of luggage, bicycles, large (I mean really large boxes), etc. If you have a question, check with your nearest post office Correos, or contact Ivar directly and ask him for advice. If the Correos will accept it for shipment, Ivar will accept it.
The "mochila transport" services are best for day-to-day, point-to-point movement of rucksacks or small luggage items along a Camino route. The Correos is best for single, long distance mailing services from wherever you are in Spain to Santiago.
But, Ivar is a very busy fellow. If you are in a hurry, my option would be to go to the Correos directly.
A third option, but you need to check first, is to ask a hotel you hold a reservation at Santiago, if they will accept and hold a parcel sent through the Correos for your arrival. I have done this on my past three Caminos for stuff I will want immediately, like clean clothes, or my shaving gear.
Ivar's service is excellent, but his "office hours" are more limited than a hotel reception desk. Now, I send things I will not want immediately to Ivar and other items, as mentioned directly to my preserved hotel.
Send your reserved hotel an e-mail using an online translator program or app. I find (IMHO) that the Microsoft or Bing translation program, or iPhone app, produces more fluid results than Google Translate. I recommend including the English text at the bottom of the e-mail message so they can try to sort out what you meant to say.
In my experience, the postal / parcel delivery services wholly within Spain are superb. Mailing from another EU country into Spain is a tad more sketchy.
I hope this helps.