Apple also offers returns via shipping. The process will just take longer. That said, I can’t say for sure if they’d require you return and reorder or if they’d let you keep the watch and just change the payment method.
On Tuesday I ordered a Series 6 SS Graphite. Scheduled Delivery 9/25-9/30. It is still on processing. I paid with my Apple Card. Should I cancel the order? Should I reorder?
Edit: Even if you do the Apple Monthly Payment option, I believe you still need to have that amount available on your Apple Card. So then your total available amount would be reduced. I think you get charged your sales tax up front. and then every month you pay monthly rate for that watch and on top of whatever balance you may have on the card.