For the best interests of your pets, only buy treatments and medications especially from your vet, not from an unknown source which is essentially what an online pharmacy is.
I'm not denigrating the online services out there and I do understand that vet bills can add up. I know, I have three cats of my own and I buy all treatments and medications from the vet, not from the pet store, online stores or even department stores.
You can never be 100% certain of what you are getting until it arrives. And when it arrives you need to be especially vigilant with the packaging and labelling of the product.
Another point is that online services can be based in many locations around the world and there are different standards and regulations between each country. There are differences between USA's and Australia's standards and legislation. What is okay in one country may not be okay in yours.
The best thing to do is to talk to your vet about your situation and between the two of you, you should be able to come to an arrangement that suits all parties.