If they dont have delivery, purchase it online and choose the Costco branch you want to pick it up from
You can have someone from within any Costco look up the item number or the item itself and find out another Warehouse has it or you can order it online if it's up there and have it delivered
I can't answer that question I'm sorry I don't know if they have mattresses or not
The website is your best option, they can deliver it (if they have it) from any of their stores. They also have a Costco Business Center, you may want to look into that as well.
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