OphanHeidi, that's actually great of Valley Vet to replace the collar! FWIW, I've heard that the Seresto collars sometimes fail if they're too loose due to not enough contact. They can also loosen themselves even if you put them on perfectly...and then they stop working. I was just having a conversation with a vet who stopped selling them due to this issue.
I also want to let people know that last week at Target I noticed really cheap sale pricing on PetArmor (Fipronil -- same active ingredient as Fronline) and Advecta3 (same active ingredients as Advantix). PetArmor also had a $4 rebate on the Ibotta app which bought its cost under $20 for 3 doses. These OTC generic dupes of name-brand products have ingredients that may or may not have ingredients that work in your area, but where they work they're very inexpensive. Just keep the Advantix/Advecta away from cats (it's not a good solution for dogs that snuggle with cats).
I just found out recently that I now need tick protection in my area due to emerging tick-borne diseases. Ticks historically haven't been a problem in the city here, but something has changed. I just dug into some research on it, and Simparica is apparently protecting against ticks for the full month, whereas Nexgard starts to drop off in the second half of the month...and Simparica is cheaper.