So I need a replacement for broken GPS cradle mount. Can I get at Costco?, no, but I can buy a new GPS for a couple hundred. At Amazon I can buy this 6 dollar item and bundled with few more low price items I can get it delivered for free.

Costco has far fewer SKUs and sell in bulk only high quality and price items. So they can afford to pay well because they only entertain customers which are spending far more for fewer high selling items and not few dollar obscure replacement parts.