Posting this as a PSA for anyone else who runs across this issue in the future and (fingers crossed) generate some momentum for the Klaviyo eng team to make an update for this edge case. 

TLDR: Linking to Costco’s website via a Klaviyo email campaign does not work unless you disable the global account setting “Email to website tracking” because of some Costco server-side security.

That setting can be disabled by going to Settings > Email > Attribution.

Longer version:

I went to send an email about a new product we have listed on Costco.com but repeatedly got server errors on Costco’s site when I tested the link out of Klaviyo. This was with the campaign UTM tracking disabled (this will be important in a second).

From Klaviyo, the Costco link is appended the following URL parameter from our account: ?_kx=ABCDEFGHIJKLMNOP%3D%3D.UHFmv9 . This is added because the global account setting "Email to website tracking" is enabled.

This URL, while correct, causes an error on the Costco servers. It seems like that error is caused by the %3D%3D in the Klaviyo URL parameter. Those characters are URL encoded "==" and likely blocked for valid security reasons. (I don’t see Costco changing these server-side security parameters any time soon based on a request from anyone external, for good reason.)

So I opened a support chat with Klaviyo and they told me to turn off the campaign UTM tracking, which was already off. After a few moments, they said the only way to turn this off is by disabling the global account setting "Email to website tracking." There is no way to disable this on the campaign level, which is obviously not great if you want any sort of data on your other email campaigns or flows that could be going out at the same time.

I opened a more formal bug report/feature request asking if Klaviyo is able to remove the %3D%3D and after about a month of back and forth was told:

The engineer got back to me and said that there are no plans at this time to remove the %3D%3D string from the _kx parameter for website tracking as we have not received many reports on this breaking for customers' websites. But they will take this feedback into account in case there are any plans on updating our tracking in the future. 

I fully recognize that asking Klaviyo to change a global account setting URL parameters for all customers is a high-risk proposition that could break all sorts of stuff, but it also feels dumb to me that any customer that wants to send a Costco link cannot do so unless they disable a global account setting.

If anyone knows of a workaround, I would love to hear it, but I’m mostly just posting this for any other poor soul that runs across this edge case.