Sorry I don't get the eBay part. So you are saying your online orders from Amazon are being shipped from Costco, Walmart etc.? I am surprised since I have not experienced that so far. I would be curious if there are others who have experienced this?
All of my shopping is done on other ecommerce sites. I usually check costco first for most things because of the customer service. Pretty much everyone offers fast, free ~2 day delivery now. The ebay part meant amazon stopped selling things themselves and went full on marketplace which is now 3rd party sellers and drop shippers. I've had several products arrive "sold by amazon" in a Walmart or Costco package. Straight up arbitrage. I complained and of course amazon customer service was great and offered to refund and add addition months on prime but never addressed my question if this was allowed. It's probably why they dropped the price matching as well. It's brilliant as a business though, they don't have to sell products at low margins, let 3rd parties sell them, and charge them for advertising their goods. Amazon will make a lot more high margin profit but as a consumer it sucks.