Wondering why my two dogs got two diffefent heartworm preventors ?

my male chi is 1 lb 11 oz and is 3 months old. he will have to go in at 6 month to repair a small umbilical hernia he has had since we got him. its nothing serious just some fatty tissue. does anyone know about an estimate as to how much this will cost? it will be fixed regardless of price. i just want to be prepared to pay for it . thanks again.