Plan it question

I need to pay for my ac to get fixed and I was gonna use my Amex and the plan it feature to help me pay it off. I saw somewhere on this subreddit that the plan it fees are typically less than the interest that would be accrued on a leftover balance so I wanted to know if I were to like pay my card as normal and before the statement closes and charges interest, can I do the plan it feature. Like I knock down the “cost” of the purchase and have the pan it payments be lower theoretically?

Btw I typically always pay my card in full each month but my cars ac going out was unplanned and I don’t have enough in the old emergency fund for it lol.