I buy the meds from petsuppliesnet.com.
Ah, there is your problem. Don't bother contacting them for the documentation; SPR will not accept it, anyway.
You are not purchasing a heartworm product made for USA, which meets FDA guidelines. You are purchasing from Australia. That is why you do not need a prescription; they have completely different rules, and different medication needs. It is also quite likely that they have a different variety of heartworm than we have here. Most importantly: you are not getting the medication strength which is required in the US; because if you were, you would only be able to get it with a prescription. It is like the difference between tylenol, and tylenol with codeine. The recommended coverage in the US for heartworm protection is the stronger version.
Normally, if I regularly dose my dogs, and then one develops heartworm anyway, the manufacturer will cover the cost of treatment; and believe me, heartworm treatment is EXPENSIVE. Even if you did keep all your receipts, if your dog did contract heartworm, Novartis would NOT cover the treatment, because you are not using the correct product for your geographic area.
If you do not buy from your vet, you need to purchase from a Vet-VIPPS certified pharmacy. The is a Veterinary-Verified Internet Pharmacy Practice site. Here is a link which explains the certification:
http://www.nabp.net/programs/accreditation/vet-vipps Drs. Foster and Smith is one. Most on line pharmacies are not. If you are not purchasing from such a place, then you are not getting the proper product. It may be counterfeit; it may be expired; in the case of the site you are using, it is designed for a completely different country, which most likely has a different variety of heartworm. So as far as SPR is concerned, you are not protecting your dogs from heartworm.
Here is the current list of Vet-VIPPS:
http://www.nabp.net/programs/accreditation/vet-vipps/find-a-vet-vipps-online-pharmacyYou WILL need a prescription, to order from any of them.
You are very fortunate that your dogs have not contracted heartworm, IMO.