It only makes sense to look at the whole picture. One agent might have the lowest fees, but the main service, shipping, is the most expensive. Same thing goes for the exchange rate: yes, a favorable exchange rate is great but it rarely makes up for expensive payment fees. This should not be up for debate. The bottom line total is the only thing that matters. Choosing the agent with the best exchange rate but paying double in service fees is not smart.
This might sound complicated and it really is: You have to figure out all the fees, and add them to the right base price in the correct order. Luckily, the RepArchive Shipping Calculator does all of that for you, it's the only resource you need for price considerations.