The main difference between spot prices and futures prices is that spot prices are instant buying and selling whereas futures contracts delay payment and delivery to a predetermined future date. Spot prices are usually lower than futures prices.
No delayed payments, mate. When you click Long/Short on a futures trade, at that time you pay the amount according to the "trading contract" with the leverage you place. And at that time the P&L will affect your futures account funds directly (depending on settings). And also there is no predetermined future date, the service only determines the "liquidation point" and "stop price point" at which your trading will be stopped automatically if you don't stop it manually.