I mentioned this in a comment to another forum post, but according to Neowin and The Verge, Costco is offering a Members-only version of the Surface Go in a bundle that includes a Surface Go with 4 GB of RAM and a 128 GB SSD (rather than 4 GB RAM / 64 GB eMMC or 8 GB RAM / 128 GB SSD). The bundle includes a Surface Type Cover and costs $549 (according to Neowin, which is $100 less than a comparable 8 GB RAM / 128 GB SSD version with Type Cover).
You do, of course, have to be a Costco member to pre-order it (for an Aug. 2 ship date). I have seen the Costco product page and folks are claiming that Microsoft has confirmed that this is a Costco-only version (and not a mis-print). Unfortunately, the comment system here won’t let me paste web links, so just Google it.