Christy and Bread are correct!
So the Merch shop will be in partnership with print on demand site Printful and for now it is limited to a select few as they iron out the bugs and any potential issues. Once the team is confident with the set up, the merch shop will roll out to all creators and made available to you once you have at least one series with 500 subscribers.
Currently the items available are mostly tops, a sweatpant, and tote. But if the program proves to be successful, more items may be added in the future!
As for the Amazon T shirt shop program, that has been discontinued for this new program.