Forget about what they say, for all you know, you've been sent the wrong product. PayPal dispute is your only option if you want a refund. Yes you can still use the product, but it's their mistake and it's not what you paid for.
I've just checked out their site, they've discontinued 54mg.. ''We have discontinued 54mg Nicotine Base as the 72mg is better value for money''.
So in my eyes, you've received one of the last batch of 54mg. Maybe it fell into the 72mg section by mistake and they didn't realize, I don't know. No way do I believe that they've got labels mixed up, if they did, then Vape Inc should no longer be selling e-liquid products.
What size bottles are you mixing, and what nicotine (mg) do you wish the achieve?