It suggests that repair is used in a more abstract environment, while repairs are used when referencing a specific instance of repair. But the verb “to fix” is much more common in everyday speech. In general, we use “repair” when we want to sound more formal. We can also use “repair” when someone with experience is doing the work.
In your OP, the latter means that you own a store where you have the tools to repair broken phones and you were going to repair your phone yourself.