ESE is an attempt by USPS to gerryrig a new service to utilize excess First Class Letter capacity by repurposing the routing bar code to tracking.
The motivation of the USPS and sellers using it is primarily financial, and it is an inferior service.
Ebay's motivation appears to have been to allow sellers of very inexpensive items to avoid the penalties associated with shipping untracked, or deciding not to offer very cheap product which cannot stand the costs of a proper tracked service.
You have discovered that you get what you pay for, and ESE is no bargain when you have to pay the fee for Below Standard performance.
I tried it, decided I was playing Russian Roulette and stopped before creating the problem you have.
Ebay cannot fix, this service. USPS cannot fix this service. You might be able to fix it by changing how you package the cards in the envelope, but every seller who uses it successfully has a different recommendation.
As a seller with a significant number of low ticket items sold, you were in a tight spot. Looks like your current limited offerings are aimed at a ticket which is more realistic for a proper shipping service, but obviously cannot reach the volume you had. And, of course, make the effort to get out from under the Below Standard penalty almost impossible for many months.
Of course, Ebay has recently increased the categories which can use ESE hoping they will find some products which behave well using it.
If someone can sell something which is a lot like a letter, they can probably safely use it.
