Can I use this product will using the Ivomec for heartworms?

I have three dogs if I order the 12 pack can I order again before the 12 packruns out

When you say that you are selling a 12 pack are you selling 12 packages with 6 chewables per pack and each package is good for 6 months? Or is your 12 pack just 2 packages with 6 chewables? I need to know for sure as I got ripped off before because I have had my dogs on Nexgard for over 2 years now and my vet is extremely expensive and I found them on line for a very reasonable price but was shocked that the price they were asking was per chewable and I did not think that those packages could be opened and sold individually, so now I am a bit scared when I find such a great deal. I need to know for sure. Also because I am ordering from Canada do I need to pay duty on this product and what would that be; could you let me know. Thanks for your time and patience.

If I order auto delivery, what happens if the price goes up? Will I be asked if I want to continue? Or be advised in any way.? Have been stung by this option once. Not interested in being stung twice :-).

I have two Frenchies between the med guidelines. They weigh 26 lbs each. Will they be safe if I get the flea meds that go to 24 libsl?