I did a test listing to check out how shipping options are showing for an item to be shipped to the US with EIS enabled. Information I read indicated that if you provide a shipping option that is lower than EIS, it will show instead of EIS. To test that I did a listing for a Blu-ray disc COO US on .ca - 25x15x3cm & 235grams. The Canada Post SBS Tracked Packet rate is $11.06.
I tested using a flat rate to the US and a calculated rate to the US. By default EIS is turned on.
Picture #1, the $14 Canada Post Tracked Packet flat rate is shown instead of the EIS rate.
Note the darn banner also shows even though it is false.
Picture #2, I revised to use calculated shipping to the US and put the weight and dimensions in the listing info. In this case, the EIS shipping rate of $19.87 is showing. I assume that is because eBay's calculated rate for other services is more than the EIS rate. I also tried a flat rate of $25 and the EIS rate of $19.87 shows instead, so the shipping costs are sorting to show the lowest rate in your listing.
Note that the blue banner is not shown when EIS shows.
Conclusion:
You can use EIS for the rest of the world, but not the USA if a flat shipping rate is chosen. So no need to opt out of EIS if your concern is shipping costs to the US. Obviously it is easy to do flat rates under 2KG with tracked packet and gets trickier for heavier items.
Listing: https://www.ebay.ca/itm/317453449861
