The sticker can be bought online. If it hasn't arrived yet then the receipt is acceptable.
In Florida you need to phone Customs at the airport of entry at least one hour prior to arrival to advise arrival time. This is in addition to eAPIS notification. You should be given the CBP officer's initials when you do. That's in case another officer when you get there can't find your 1 hr notification so you have some way to show you complied.