As explained by @Richard.W, the fee is applied through adjustment of the exchange rate. Here is an example:
**Breakdown of an example calculation of how applying the fee looks like
| Direction | Spot GBP/USD rate | FX Conversion rate |
|---|---|---|
| 1.4000 / 1.4100 | ||
| SELL | 0.15/100 x 1.4 = 0.0021 | |
| 1.4000 - 0.0021 = 1.3979 | 1.3979 | |
| BUY | 0.15/100 x 1.4100 = 0.002115 | |
| 1.4100 + 0.002115 = 1.412115 | 1.412115 |