Ran into the same "issue" starting last week or so. I contacted ebay and they said they were rolling this
out in waves and randomly selecting buyers to implement it on their account. I ask if there was any way
to get this removed and they said no.

Here's an article on it that says just about the same thing: https://www.ecommercebytes.com/C/blog/blog.pl?/comments/2023/8/1691968145.html/1/0

It's great for sellers on probably 95% of items (they don't have to wait for a buyer to pay up to 4 days later)
but it's going to be a huge pain for buyers (and less sellers) of items like chips/token/dice etc. Really
anything where it's common to purchase more than one item/listing from a single buyer specifically to
benefit from combined shipping.

IIRC something like this initially occurred in 2021 but was either paused or rolled back.