If I shiver I keep shivering more and more until I get to the warmer place, drink something hot, or put more cloth on. I NEVER quit shivering just because I do it, just feel more miserable with time.
Personally if your horse shivers I'd put a medium weight blanket on or at least a waterproof sheet to protect from the wind and snow (especially if it doesn't stay in shelter). BTW my cheap Saxon blankets ($40/new) last my mares for the 5th winter already.
Can you pay extra to the BO to take on/off the blanket and do little brushing when it's off? That would be ideal.
"Life isn't about waiting for the storm to pass: it's about learning to dance in the rain..."
"When we are no longer able to change a situation - we are challenged to change ourselves."
"How people treat you is their karma; how you react is yours."