。
I'd like a cold one, please.
Literal
Cold [thing-の] [object-を] please-ください.
冷やしたの = 'one that has been cooled,' where 冷やした is the past of 冷やす ('to cool something') and の is the nominalizer ('one'). This の-nominalizer is highly productive for 'the X one' constructions. A natural request at a counter.