Sorry if I missed the explanation for this in an earlier grammar note, but isn't "platit" an imperfective verb, so "Já už jsem platila" would mean "I was already paying", whereas "já už jsem zaplatil" means "I have already paid"?
I added the "I was already paying" version. But that is an unlikely meaning of the given sentence, which actually comes from a classic movie about a fake waiter.
Imperfective verbs are often used to refer even to completed actions.
Why is jsem not in the second slot in this sentence? Does už also compete for it and win?
Yes, both jsem and už are in the second position here. Most often the order of jsem and už can be switched as they have a similar priority level.