K.B.
I think any company that will sell medication without a prescription is rather sketchy. Yes, the company is in Australia, and they don't necessarily have the same standards as we do here in the U.S. The products may not be effective because they may have not been stored properly and the companies that make them do NOT guarantee their effectiveness if they are purchased from an on-line pharmacy. In other words, if someone buys heartworm prevention at their vet, administers it correctly, and their dog still gets heartworm, the company will often pay for having the dog treated. If the medication is from an on-line pharmacy, they don't guarantee it's safety or efficacy, so that's the chance people take. You can try getting it from them, but I've always seen requests get faxed to the hospitals I have worked at and we can still approve or not approve it, if the pet has not been tested recently.
I would look into low-cost alternatives, like Momma L. suggested. A lot of practices now are doing heartworm clinics this time of year since it's "the busy season" and may be testing at a reduced cost. Also, some practices will price-match the PetMeds Express price, just so the medication is still purchased through them.