Medication for puppies?

Our puppy has been with us for a month. At the first visit the vet gave us a free sample of heartguard and another for sentinal. she said that since sentinal worked for both heartworm and fleas we could give out puppy that and repeat the process on same day every month. Now we meant to order more sentinal for the coming months but forgot and now the 4 weeks are up. and petsmart said they dont carry medication. so our only choice is to order sentinal online which will take 4-5 business days. My question:

1-will not giving the med on the exact date harm the puppy?

2-if its essential to give the med on the exact same day then can I give the other sample heartguard med to her this month and switch back to sentinal next month?


Please could anybody knowledgeable answer! .....would be v grateful