I’m sorry you had this experience.
You can file a dispute in the resolution center (found in the upper right corner of the order). Cancellations will affect your Gig and may negatively impact sales. So far I’ve had to cancel a few orders during my time on Fiverr, and it hasn’t created any catastrophic fall in sales.
However, it’s always best to solve this type of issue via communication, but since your buyer isn’t responding you should, in my opinion, fila a dispute for the order. You can offer the buyer to pay the right price, or to go for a mutual cancellation.
If the buyer does not respond within a few days to the dispute, the order will be automatically cancelled.
I know some buyers will make small orders and then request additional work to be done, expecting the seller to provide it in fear of a negative review or late delivery. They will ignore the buyer after placing the order, and see what happens. Some buyers will get scared and do the work - in short, they will work for next to nothing.
The buyer has nothing to loose anyway, because they will get their work done, or money back. It’s a nasty practice done by dishonest people.
The Resolution Center will solve this for you
Best of luck!