回一下供其他人參考好了。
我2017/8寄信給美國好市多問過，
掐去個資回應如下：
系統認不出海外會員的帳號。
所以先網購，再寄信給他們告知卡號跟訂單編號，他們會退5%的非會員使用費。
在實體賣場則可以直接使用海外會員卡。
At this time, Costco.com does not recognize International memberships.To shop on Costco.com, you will need to exclude your membership number from your account information located in they Accountection of our site. Please do not enter your membership number at the time of registration and/or checkout. As long as you have a U.S. shipping and billing address to use on your order, you are still able to place an order.
f you are charged a 5% non-member surcharge fee when placing your order, pleaseontact uso we can refund the surcharge fee. We will need your order number and your current membership number to process the refund back to your original method of payment used on your order.
f you have recently relocated to the U.S. and need to change your International membership to a U.S. membership, please visit yourlosest Costco warehouse. The warehouse has the ability to assist you.You may use your membership in person at any warehouse as well, the restriction is for online only.
I hope this information has been helpful Lenore!Do let us know if we can be of any further assistance.

推實證文

推～

請問那註冊線上會員時要求會員編號怎麼解？(還是說現在不用了?)

美國costco註冊會員不用會員編號 因為不是會員也可以註冊跟購買 就如同本文所說的 非會員會加5%

剛原本要跟美國Costco退5%， 被拒絕了！ 國外會员卡可以到美國使用， 但是5%必須是美國的會员卡， 他幫我在系统輸入台灣會員卡卡號無法退款！

這篇回信是說，在註冊及結帳時都"不用"輸入海外卡的卡號系統會先收取5%非會員手續費，之後自行向線上客服聯絡，並提供訂單號碼及海外卡的卡號，客服會手動退還手續費至原始的付款方式上。