What's the difference between 少{すこ}し (sukoshi) and 小{ちい}さい (chiisai)?
In what situations would I use each one?
To add to the answer from Ignacio, 小さい is "little" that is opposite to "big" while 少し is "a little" that is opposite to "a lot". That is to say that 小さい describes the small size/volume, while 少し describes the small quantity/magnitude.
Adding 少し in "もう少し安いのはありませんか" adds the connotation of "a little" to "cheaper" to become "Is there one that is a little cheaper?" instead of simply "Is there one that is cheaper?". It softens the tone of the request.
少し
means 'less'. もう少し
means 'a little more'.
「少し」 is an adverb, and as such it modifies verbs (「少し話せる」 -> "can speak a little"), and refers to ability or capability. 「小さい」 is an adjective, and so modifies nouns (「小さい餓鬼」 -> "little brat"), and refers to size.