I just ordered a silver SS watch 7, it has the shipping day of early December. I chose the Apple Card monthly payment plan, they charged me tax on the watch today, but do they start charging the monthly payment after I receive the watch?
My own experiences have only been with products that had no delays in shipping, but I'm pretty sure you won't be charged until the watch is either shipped or received, probably shipped. Difference is only a couple of days.
Pending charges for taxes have always shown in Wallet right away for me. But actually being charged for taxes when the item ships has been my experience so far. The installment won't start until it ships.