First off, I wouldn't use those drops, since you're not sure they are authentic Advantage flea treatment, regardless of how well they are sealed or professionally packaged they look.
Ask your vet to check the packaging and origin, etc. to see if they are really authentic or something that might make your cat/s ill. Sadly, this happens at times...
Next, if they do prove to be authentic, see them as a free gift. You didn't order them, but they were sent unsolicited. So, free...
Finally, I used Front Line flea drops in past years, switched to Advantage this year (different species of flea this year, I think), and finally settled on a much cheaper and equally effective Blue Bottle flea treatment, available through Amazon.com. Front Line and Advantage are rather expensive when you have as many cats as we have (about 30 rescued cats) and Blue Bottle was as effective and much more cost-effective (about $50 for more than 70 1-month treatments).
Just type "Blue Bottle Flea Treatment" in the Amazon search engine.