I had the same issue when I joined. The pricing is not clear. I will attempt explaining it. You are required to pay a premium trading charge for the first month after joining. So, if this is your first month the charges are normal.
Subsequent months will be £3 for dealing UK shares and $0 for US shares if you meet the set requirements. You have to at least place 3 trades monthly. If you don’t achieve this requirement you will pay more.
You also have to bear in mind; the pricing model is different if you have a multi currency account. The requirement stated above is not applicable and you will pay more. To trade instruments across multiple exchanges, do the maths and subscribe to the cheapest option. Options for currency conversions are manual and instant.
To ensure your qualify for the discounted rate, you must have your current conversion set to instant. With regards to your calculation, convert the total cost to dollars to identify the 0.5%.
For more details read this link https://www.ig.com/uk/investments/share-dealing/costs-fees