"Penny" weighs 20.1 pounds, and the 11-25 pound dose is the one size they have in stock at allivet.com. I called my vet, and Allivet is on their list of approved online pharmacies. So I ordered 2 boxes really quick, which will last this year and most of next year.
I could have ordered the other dogs' Heartgard chews too and saved a few bucks AND qualified for free shipping. OTOH I don't have their prescriptions yet, and Allivet could easily run out of Interceptor before I could get my paperwork together.