Exchange rate margin
An exchange rate margin is the gap between the mid-market rate and the rate a provider actually gives you, and it is a cost even when the provider advertises no fee.
The mid-market rate is the midpoint between what buyers and sellers are quoting for a currency pair. It is the rate you see on a search engine, and it is not a rate anybody transacts at.
A provider that quotes you a worse rate is charging you the difference. On a $1,000 transfer, a 2% margin costs $20 — which can be more than a provider charging a visible $5 fee at a better rate.
The only way to compare two providers honestly is to compare the amount the recipient ends up with, for the same amount sent, at the same moment. Fee and rate cannot be compared separately.