Sling Money does not mark-up the rate of exchange between currencies. For most currencies, we use the "mid-market" rate, as published by CurrencyAPI.
We may use an exchange rate provided by our local partners for certain currencies with less liquidity in the market. This is the case, for example, for most African currencies and some currencies in Latin America.
When using a rate set by one of our partners, we do not mark up the rate. We always negotiate the best available rates with our partners to pass those savings on to our users directly.