^Agree with everything OHT said.
The product you linked to is a heartworm preventative based on the ingredient moxidectin. Apparently it is widely relied upon in other countries but not so much in the U.S. According to this page -- http://www.marvistavet.com/html/body_heartworm_prevention.html -- the product Proheart 6 was voluntarily off the U.S. market from 2004 to 2008 due to suspected adverse reactions in pets.
Of course any oral, injected or topical treatment can cause an adverse reaction in pets. But I might ask my vet about this product before ordering it, just to get his/her OK.
Heartworm prevention is sadly NOT cheap, I know. If it were me, I'd probably go with a product that I already have experience with and shop around for the best price on it. For me, those products are Heartgard or Sentinel.
Also, I note on that website you linked to, that if you click on the Proheart products to buy them, you're taken to an entirely different website to actually shop.
Last, I think I saw somewhere on there that they'd sell the product to you without a prescription. Which might be fine, except the reason for only being able to obtain heartworm preventative in the U.S. with a prescription is because a vet will want to test your dog for heartworms before administering a preventative. If a preventative is unknowingly administered to a dog who already has heartworms, it can cause serious complications and even death.