2D texture for 2 seconds on screen?

You could create a GuiTexture. Position it where you want it. Then:

 var texture : Texture2D;
 var On:boolean;
 
 function Start(){
      guiTexture.enabled = false;
 }
 
 function Update(){
   if(coins==200){
      On=true}}
   if(On){
      GuiGoesOn();}

 function GuiGoesOn(){
    On=!On;
    guiTexture.enabled=true;
    yield WaitForSeconds(2);
    guiTexture.enabled=false;
 }

Now is your coin counting in the same script? Is it a static variable?If yes, call the static with

 ScriptName.countVar

If not static, you still can send a message from the script where the counting happens. Somehow like this

in the GuiTexture script

 var texture : Texture2D;
 
 function Start(){
     guiTexture.enabled = false;
 }
 
  function GuiGoesOn(){
     guiTexture.enabled=true;
     yield WaitForSecond(2);
     guiTexture.enabled=false;
    }

In the script where coins are counted:

 function Update(){
      if(coins == 200)
      {
          On = True; 
      }
      if(On){
             gameObject.Find("TheGUItextureObject").SendMessage("GuiGoesOn");
             On=!On;
      }  
 }

I edited a little as I realized that it could work but would not be the best.