I have seen several places that sell pet food online, but unless you buy a lot of it (or it is really expensive pet food), there are always high shipping costs, and the selection is usually not that great either.
Most of the places that I have checked out have been for one of the more expensive brands of dog food, and I don't feed that to my dog anyway.
Since I am almost 70 years old now, it has also gotten a lot harder for me to carry in the dog food when I buy the dog food at the store, and the large economy size is simply impossible. I struggle with the little 20 lb bags anymore.
So, I started looking for affordable online dog food, and found it in my already favorite shopping place ----- AMAZON ! !
Yes, Amazon carries all kinds of dog food, the prices are cheaper than my feed store, or the grocery store, I don't have to get a 50lb bag like at Sam's Club, and best of all, Amazon doesn't charge shipping or tax, and delivers it to my front door in two days.
I now have an Amazon Prime account, I can get free 2-day shipping on almost everything I order, and not have to pack it home and unload it myself.
If you are thinking of buying your dog food online, I recommend checking this out.