You will be charged $7 total to enter and then exit a one-standard-lot (100K) position.
This is referred to as the “round-turn commission”, covering entry ($3.50) and exit ($3.50). Since you have to get out of every position you get into, the round-turn commission (plus the applicable spread) represents the total transaction cost associated with your trade.
The $3.50 figure represents the commission cost “per leg” – one leg to get into your position, and one leg to get out. It’s a useless figure. Focus on the round-turn commission.
That round-turn commission is pro-rated according to your actual position size. So, if you trade one mini-lot (10K units = 0.1 Lot), then the round-turn commission charged to your trade will be $0.70 (one-tenth of the one-lot commission). If you trade one micro-lot (1K units = 0.01 Lot), the round-turn commission will be $0.07.
The position sizes mentioned above are the (leveraged) notional amounts traded, regardless of how much required margin is associated with your account.