Well, I see it this way.

RP and MC have Small Rats for $1.50 (standard rate; not counting the special sales they sometimes have). So I usually get 100 at a time. So that's $150 + ~$45 shipping = $195ish.

ARS is $1.75. Times 100 = $175 + No shipping. So I save $20 there. That's a little more that 10% savings.