Get a $20 voucher to PetCareRX for only $1.99 at OnSale! You can spend this on anything at their site including, pet medications, flea treatments, and various pet supplies. You save 90% and you can avoid shipping fees by spending $35 or more. Spending less than $35 will cost you $5.99 in shipping but the deal is still pretty good even then!
UPDATE: When you go to use your voucher to PetCareRX, be sure to go through Mr. Rebates so that you can earn 12% cash back! This deal is amazing!