I make a spread sheet each time I have to order. I include:
website; cost; shipping

I put in a total in the last column.

Due to the number of pups here, I order the 12 packs, so I don't need to keep track of 6 packs, 3 packs. If the web site doesn't offer the 12 pack, I don't even list it. I buy at least a 3 month supply each time.

I find it varies a bit each time I have to order, as to which site has the best deal on offer.

I have used:
Entirely Pets
2 different companies based in Australia (I know, supposedly this is a different chemical line up; considering the savings, I went ahead anyhow).