Sofia to Skopje?
 

Can anyone tell me if any airline flies Sofia to Skopje on the following dates:

May 3 outbound
May 10 return

I do not want to take the bus, and there is no train service..

Thanks!

There are no direct flights, but you can fly via PRG (or BUD outside Skyteam).
For your dates, CSA will sell you a through ticket for 467 Euros, which may not be cheap, but it's still infinitely better than a bus. MA is cheaper, but still not cheap (EUR343).

If I were you, and given that you are returning to SOF, I would also think about renting a car. I'm pretty sure Sixt will allow you to take the car abroad (as long as you return it to where you rented it from).