回一下供其他人参考好了。
我2017/8寄信给美国好市多问过，
掐去个资回应如下：
系统认不出海外会员的帐号。
所以先网购，再寄信给他们告知卡号跟订单编号，他们会退5%的非会员使用费。
在实体卖场则可以直接使用海外会员卡。
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.

推实证文

推～

请问那注册线上会员时要求会员编号怎么解？(还是说现在不用了?)

美国costco注册会员不用会员编号 因为不是会员也可以注册跟购买 就如同本文所说的 非会员会加5%

刚原本要跟美国Costco退5%， 被拒绝了！ 国外会员卡可以到美国使用， 但是5%必须是美国的会员卡， 他帮我在系统输入台湾会员卡卡号无法退款！

这篇回信是说，在注册及结帐时都"不用"输入海外卡的卡号系统会先收取5%非会员手续费，之后自行向线上客服联络，并提供订单号码及海外卡的卡号，客服会手动退还手续费至原始的付款方式上。