Shipping clothes to and from Barcelona

It would probably be cheaper to pay to check an extra bag on the way home than to ship it. Or enquire with your airline about sending it freight.

Better idea, plan really carefully with layers and just bring one bag...

I don't think what you are proposing makes much sense. We pack very close to the same items whether it is summer or winter. The secret is smart use of laying that voids the need for the second bag. Shipping the second bag and would be expensive (probably in the range of 50/75 $ ) to ship it. It is just not worth it. And storing it somewhere would also be very expensive. You cannot expect a hotel to hold on to your stuff for three weeks. This item needs a little rethinking.

I agree with the others, plan your wardrobe with some layers. It's not likely to be that much colder in early October than in September even in northern Spain. You'll need some kind of waterproof or water repellent shell for the possibility of rain in both Sept and Oct and adding a fleece vest and a light weight long sleeve shirt for underneath would probably be enough unless you're going to be hiking high in the mountains. If you need anything add'l you could probably find something inexpensive over there that would be cheaper than either storing or shipping an add'l bag.

Storage for three weeks may be considerend at a train station or the airport, but even at a conservative price it will be over $150, then the shipping home, or extra $100 for an extra suitcase. It starts to cost to be a fashionista. Packing light and accepting that laundry needs doing helps the pocketbook and is more convenient for getting up tight stairwells, getting on and off trains with short stops, cobble streets, renting a car (trunks are very small).
Precise clothing choices allow transition in temperatures. A lightweight stuffable down jacket or vest weights little, takes up little space and allows a layer for colder days. Also a fine weight merino wool cardigan or sweater for inbetween weather and you are covered.

I have left two large empty bike boxes at my first hotel stay for 3-4 weeks. Looked for hotel that stated stored luggage and then verified through email if it was ok. They would be place in a locked closet or behind check in desk. The locked closet would be a better choice for clothes. The boxes were marked with out name or we were given a hat check ticket. For a long trip it sure would be nice to have a second suitcase!

I also would not bring a lot of extra clothes for the northern part of Spain in October. If it's not all manageable in one bag when you depart, it won't be much fun to haul around during the northern part of the trip.

But if there are a few items that you stuff into your bag at the start and then don't need access to during the majority of the tour, ask your tour manager and driver about leaving a bag in "deep storage" under the bus during the bus portion of the trip, from Madrid to Sevilla. Your deep storage bag would probably be a light, small one that you can use as an airline "personal item," such as Rick's Hide-Away Tote or Euro Flight Bag.

P.S. Edited - sorry if you're getting multiple emails.